constructs are now importable from the "from sqlalchemy" namespace,
just like every other Core construct.
- The implicit conversion of strings to :func:`.text` constructs
when passed to most builder methods of :func:`.select` as
well as :class:`.Query` now emits a warning with just the
plain string sent.   The textual conversion still proceeds normally,
however.  The only method that accepts a string without a warning
are the "label reference" methods like order_by(), group_by();
these functions will now at compile time attempt to resolve a single
string argument to a column or label expression present in the
selectable; if none is located, the expression still renders, but
you get the warning again. The rationale here is that the implicit
conversion from string to text is more unexpected than not these days,
and it is better that the user send more direction to the Core / ORM
when passing a raw string as to what direction should be taken.
Core/ORM tutorials have been updated to go more in depth as to how text
is handled.
fixes #2992

- add support for PG INHERITS
- fix mis-named tests
- changelog
fixes #2051

- return a list of dicts like other methods do
- don't combine 'schema' with 'name', leave them separate
- support '*' argument so that we can retrieve cross-schema
if needed
- remove "conn" argument
- use bound parameters for 'schema' in SQL
- order by schema, name, label
- adapt _load_enums changes to column reflection
- changelog
- module docs for get_enums()
- add drop of enums to --dropfirst

Provide opportunity to get enums list via an inspector instance public
interface.

otherwise render a SQL NULL column value, rather than a JSON-encoded
``'null'``.  To support this case, changes are as follows:

* The value :func:`.null` can now be specified, which will always
  result in a NULL value resulting in the statement.

* A new parameter :paramref:`.JSON.none_as_null` is added, which
  when True indicates that the Python ``None`` value should be
  peristed as SQL NULL, rather than JSON-encoded ``'null'``.

Retrival of NULL as None is also repaired for DBAPIs other than
psycopg2, namely pg8000.

fixes #3159

non-standard DBAPI exceptions, such as the psycopg2
TransactionRollbackError.  These exceptions will now be raised
using the closest available subclass in ``sqlalchemy.exc``, in the
case of TransactionRollbackError, ``sqlalchemy.exc.OperationalError``.
fixes #3075