Change-Id: I625af65b3fb1815b1af17dc2ef47dd697fdc3fb1

Adds support for comments on named constraints, including `ForeignKeyConstraint`, `PrimaryKeyConstraint`, `CheckConstraint`, `UniqueConstraint`, solving the [Issue 5667](https://github.com/sqlalchemy/sqlalchemy/issues/5667).

Supports only PostgreSQL backend.

### Description

Following the example of [Issue 1546](https://github.com/sqlalchemy/sqlalchemy/issues/1546), supports comments on constraints. Specifically, enables comments on _named_ ones — as I get it, PostgreSQL prohibits comments on unnamed constraints.

Enables setting the comments for named constraints like this:
```
Table(
   'example', metadata,
   Column('id', Integer),
   Column('data', sa.String(30)),
   PrimaryKeyConstraint(
       "id", name="id_pk", comment="id_pk comment"
    ),
   CheckConstraint('id < 100', name="cc1", comment="Id value can't exceed 100"),
   UniqueConstraint(['data'], name="uc1", comment="Must have unique data field"),
)
```

Provides the DDL representation for constraint comments and routines to create and drop them. Class `.Inspector` reflects constraint comments via methods like `get_check_constraints` .
### Checklist
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This pull request is:

- [ ] A documentation / typographical error fix
- [ ] A short code fix
- [x] A new feature implementation
	- Solves the issue 5667.
	- The commit message includes `Fixes: 5667`.
	- Includes tests based on comment reflection.

**Have a nice day!**

Fixes: #5667
Closes: #7742
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7742
Pull-request-sha: 42a5d3c3e9ccf9a9d5397fd007aeab0854f66130

Change-Id: Ia60f578595afdbd6089541c9a00e37997ef78ad3

still can't figure out the warnings with some of the older
changelog files.

this cherry-picks the sphinx fixes from 1.4 and additionally
fixes a small number of new issues in the 2.0 docs. However,
2.0 has many more errors to fix, primarily from the removal
of the legacy tutorials left behind a lot of labels that need
to be re-linked to the new tutorial.

Fixes: #7946
Change-Id: Id657ab23008eed0b133fed65b2f9ea75a626215c
(cherry picked from commit 9b55a423459236ca8a2ced713c9e93999dd18922)

implement strict typing for schema.py

this module has lots of public API, lots of old decisions
and very hard to follow construction sequences in many
cases, and is also where we get a lot of new feature requests,
so strict typing should help keep things clean.

among improvements here, fixed the pool .info getters
and also figured out how to get ColumnCollection and
related to be covariant so that we may set them up
as returning Column or ColumnClause without any conflicts.

DDL was affected, noting that superclasses of DDLElement
(_DDLCompiles, added recently) can now be passed into
"ddl_if" callables; reorganized ddl into ExecutableDDLElement
as a new name for DDLElement and _DDLCompiles renamed to
BaseDDLElement.

setting up strict also located an API use case that
is completely broken, which is connection.execute(some_default)
returns a scalar value.   This case has been deprecated
and new paths have been set up so that connection.scalar()
may be used.  This likely wasn't possible in previous
versions because scalar() would assume a CursorResult.

The scalar() change also impacts Session as we have explicit
support (since someone had reported it as a regression)
for session.execute(Sequence()) to work.  They will get the
same deprecation message (which omits the word "Connection",
just uses ".execute()" and ".scalar()") and they can then
use Session.scalar() as well.  Getting this to type
correctly while still supporting ORM use cases required
some refactoring, and I also set up a keyword only delimeter
for Session.execute() and related as execution_options /
bind_arguments should always be keyword only, applied these
changes to AsyncSession as well.

Additionally simpify Table __init__ now that we are Python
3 only, we can have positional plus explicit kwargs finally.
Simplify Column.__init__ as well again taking advantage
of kw only arguments.

Fill in most/all __init__ methods in sqltypes.py as
the constructor for types is most of the API.   should
likely do this for dialect-specific types as well.

Apply _InfoType for all info attributes as should have been
done originally and update descriptor decorators.

Change-Id: I3f9f8ff3f1c8858471ff4545ac83d68c88107527

__future__.annotations mode allows us to use non-string
annotations for argument and return types in most cases,
but more importantly it removes a large amount of runtime
overhead that would be spent in evaluating the annotations.

Change-Id: I2f5b6126fe0019713fc50001be3627b664019ede
References: #6810

large patch to get ORM / typing efforts started.
this is to support adding new test cases to mypy,
support dropping sqlalchemy2-stubs entirely from the
test suite, validate major ORM typing reorganization
to eliminate the need for the mypy plugin.

* New declarative approach which uses annotation
  introspection, fixes: #7535
* Mapped[] is now at the base of all ORM constructs
  that find themselves in classes, to support direct
  typing without plugins
* Mypy plugin updated for new typing structures
* Mypy test suite broken out into "plugin" tests vs.
  "plain" tests, and enhanced to better support test
  structures where we assert that various objects are
  introspected by the type checker as we expect.
  as we go forward with typing, we will
  add new use cases to "plain" where we can assert that
  types are introspected as we expect.
* For typing support, users will be much more exposed to the
  class names of things.  Add these all to "sqlalchemy" import
  space.
* Column(ForeignKey()) no longer needs to be `@declared_attr`
  if the FK refers to a remote table
* composite() attributes mapped to a dataclass no longer
  need to implement a `__composite_values__()` method
* with_variant() accepts multiple dialect names

Change-Id: I22797c0be73a8fbbd2d6f5e0c0b7258b17fe145d
Fixes: #7535
Fixes: #7551
References: #6810

Change-Id: I49abf2607e0eb0623650efdf0091b1fb3db737ea

Change-Id: I1ef2eb2018f4b68825fe40a2a8d99084cf217b35
References: #7257

Also replace http://pypi.python.org/pypi with https://pypi.org/project

Change-Id: I84b5005c39969a82140706472989f2a30b0c7685

Change-Id: Ic5bb19ca8be3cb47c95a0d3315d84cb484bac47c