Sqlacodegen

- Dropped support for Python < 3.7
- Dropped support for SQLAlchemy 1.3
- Added a ``__main__`` module which can be used as an alternate entry point to the CLI
- Added detection for sequence use in column defaults on PostgreSQL
- Fixed ``sqlalchemy.exc.InvalidRequestError`` when encountering a column named
"metadata" (regression from 2.0)
- Fixed missing ``MetaData`` import with ``DeclarativeGenerator`` when only plain tables
are generated
- Fixed invalid data classes being generated due to some relationships having been
rendered without a default value
- Improved translation of column names into column attributes where the column name has
whitespace at the beginning or end
- Modified constraint and index rendering to add them explicitly instead of using
shortcuts like ``unique=True``, ``index=True`` or ``primary=True`` when the constraint
or index has a name that does not match the default naming convention

- Fixed ``IDENTITY`` columns not rendering properly when they are part of the primary
key

**NOTE**: Both the API and the command line interface have been refactored in a
backwards incompatible fashion. Notably several command line options have been moved to
specific generators and are no longer visible from ``sqlacodegen --help``. Their
replacement are documented in the README.

- Dropped support for Python < 3.6
- Added support for Python 3.10
- Added support for SQLAlchemy 1.4
- Added support for bidirectional relationships (use ``--option nobidi``) to disable
- Added support for multiple schemas via ``--schemas``
- Added support for ``IDENTITY`` columns
- Disabled inflection during table/relationship name generation by default
(use ``--option use_inflect`` to re-enable)
- Refactored the old ``CodeGenerator`` class into separate generator classes, selectable
via ``--generator``
- Refactored several command line options into generator specific options:

- ``--noindexes`` → ``--option noindexes``
- ``--noconstraints`` → ``--option noconstraints``
- ``--nocomments`` → ``--option nocomments``
- ``--nojoined`` → ``--option nojoined`` (``declarative`` and ``dataclass`` generators
only)
- ``--noinflect`` → (now the default; use ``--option use_inflect`` instead)
(``declarative`` and ``dataclass`` generators only)
- Fixed missing import for ``JSONB`` ``astext_type`` argument
- Fixed generated column or relationship names colliding with imports or each other
- Fixed ``CompileError`` when encountering server defaults that contain colons (``:``)

- Added support for rendering computed columns
- Fixed ``--nocomments`` not taking effect (fix proposed by AzuresYang)
- Fixed handling of MySQL ``SET`` column types (and possibly others as well)

- Added support for rendering table comments (PR by David Hirschfeld)
- Fixed bad identifier names being generated for plain tables (PR by softwarepk)

- Dropped support for Python 3.4
- Dropped support for SQLAlchemy 0.8
- Added support for Python 3.7 and 3.8
- Added support for SQLAlchemy 1.3
- Added support for column comments (requires SQLAlchemy 1.2+; based on PR by koalas8)
- Fixed crash on unknown column types (``NullType``)