Improved support for column names that contain percent signs in the string,
including repaired issues involving anoymous labels that also embedded a
column name with a percent sign in it, as well as re-established support
for bound parameter names with percent signs embedded on the psycopg2
dialect, using a late-escaping process similar to that used by the
cx_Oracle dialect.

* Added new constructor for _anonymous_label() that ensures incoming
  string tokens based on column or table names will have percent
  signs escaped; abstracts away the format of the label.

* generalized cx_Oracle's quoted_bind_names facility into the compiler
  itself, and leveraged this for the psycopg2 dialect's issue with
  percent signs in names as well.  the parameter substitution is now
  integrated with compiler.construct_parameters() as well as the
  recently reworked set_input_sizes(), reducing verbosity in the
  cx_Oracle dialect.

Fixes: #5653

Change-Id: Ia2ad13ea68b4b0558d410026e5a33f5cb3fbab2c

Reworked the "setinputsizes()" set of dialect hooks to be correctly
extensible for any arbirary DBAPI, by allowing dialects individual hooks
that may invoke cursor.setinputsizes() in the appropriate style for that
DBAPI.   In particular this is intended to support pyodbc's style of usage
which is fundamentally different from that of cx_Oracle.  Added support
for pyodbc.

Fixes: #5649
Change-Id: I9f1794f8368bf3663a286932cfe3992dae244a10

The server_side_cursors engine-wide feature relies upon
regexp parsing of statements a well as general guessing as
to when the feature should be used.   This is not within the
2.0 way of doing things and should be removed.

Additionally, mariadbconnector defaults to unbuffered cursors;
add new cursor hooks so that mariadbconnector can specify
buffered or unbuffered cursors without too much difficulty.
This will also correctly default mariadbconnector to buffered
cursors which should repair the segfaults we've been getting.

Try to restore the assert_raises that was removed in
5b6dfc0c38bf1f01da4b8 to see if mariadbconnector segfaults
are resolved.

Change-Id: I77f1c972c742e40694972f578140bb0cac8c39eb

Make optional sequences render as identity in mssql
Remove unused dialect option sequence_default_column_type

Change-Id: I821eeffcb442f8d1b69186a9b798b15c3d8d6ff3

This change includes mainly that the bracketed use within
select() is moved to positional, and keyword arguments are
removed from calls to the select() function.  it does not
yet fully address other issues such as keyword arguments passed
to the table.select().

Additionally, allows False / None to both be considered
as "disable" for all of select.correlate(), select.correlate_except(),
query.correlate(), which establishes consistency with
passing of ``False`` for the legact select(correlate=False)
argument.

Change-Id: Ie6c6e6abfbd3d75d4c8de504c0cf0159e6999108

Added support for PostgreSQL "readonly" and "deferrable" flags for all of
psycopg2, asyncpg and pg8000 dialects.   This takes advantage of a newly
generalized version of the "isolation level" API to support other kinds of
session attributes set via execution options that are reliably reset
when connections are returned to the connection pool.

Fixes: #5549
Change-Id: I0ad6d7a095e49d331618274c40ce75c76afdc7dd

MariaDB should not run a Sequence if it has optional=True.
Additionally, rework the rules in crud.py to accommodate the
new combination MariaDB brings us, which is a dialect
that supports both cursor.lastrowid, explicit sequences,
*and* no support for returning.

Co-authored-by: Mike Bayer <mike_mp@zzzcomputing.com>
Fixes: #5528
Change-Id: I9a8ea69a34983affa95dfd22186e2908fdf0d58c

Improved the :func:`_sql.tuple_` construct such that it behaves predictably
when used in a columns-clause context.  The SQL tuple is not supported as a
"SELECT" columns clause element on most backends; on those that do
(PostgreSQL, not surprisingly), the Python DBAPI does not have a "nested
type" concept so there are still challenges in fetching rows for such an
object. Use of :func:`_sql.tuple_` in a :func:`_sql.select` or
:class:`_orm.Query` will now raise a :class:`_exc.CompileError` at the
point at which the :func:`_sql.tuple_` object is seen as presenting itself
for fetching rows (i.e., if the tuple is in the columns clause of a
subquery, no error is raised).  For ORM use,the :class:`_orm.Bundle` object
is an explicit directive that a series of columns should be returned as a
sub-tuple per row and is suggested by the error message. Additionally ,the
tuple will now render with parenthesis in all contexts. Previously, the
parenthesization would not render in a columns context leading to
non-defined behavior.

As part of this change, Tuple receives a dedicated datatype
which appears to allow us the very desirable change of removing
the bindparam._expanding_in_types attribute as well as
ClauseList._tuple_values (which might already have not been
needed due to #4645).

Fixes: #5127
Change-Id: Iecafa0e0aac2f1f37ec8d0e1631d562611c90200

Using the approach introduced at
https://gist.github.com/zzzeek/6287e28054d3baddc07fa21a7227904e

We can now create asyncio endpoints that are then handled
in "implicit IO" form within the majority of the Core internals.
Then coroutines are re-exposed at the point at which we call
into asyncpg methods.

Patch includes:

* asyncpg dialect

* asyncio package

* engine, result, ORM session classes

* new test fixtures, tests

* some work with pep-484 and a short plugin for the
  pyannotate package, which seems to have so-so results

Change-Id: Idbcc0eff72c4cad572914acdd6f40ddb1aef1a7d
Fixes: #3414