Remove pin on SQLAlchemy version < 2.0

[jace]

WTForms-SQLAlchemy with SQLAlchemy 2.0 is known to be working.

[azmeuk]

CI is failing. Under normal circumstances, there is at least one SQLAlchemy 2 deprecation warning raised when running the tests.

tests/test_main.py::QuerySelectFieldTest::test_with_query_factory
  /home/eloi/dev/wtforms/wtforms-sqlalchemy/tests/test_main.py:77: RemovedIn20Warning: Deprecated API features detected! These feature(s) are not compatible with SQLAlchemy 2.0. To prevent incompatible upgrades prior to updating applications, ensure requirements files are pinned to "sqlalchemy<2.0". Set environment variable SQLALCHEMY_WARN_20=1 to show all deprecation warnings.  Set environment variable SQLALCHEMY_SILENCE_UBER_WARNING=1 to silence this message. (Background on SQLAlchemy 2.0 at: https://sqlalche.me/e/b8d9)
    mapper(Test, test_table)

[jace]

I'll review and push a fix tonight.

[jace]

SQLAlchemy 1.4 is now the minimum version as the location of declarative_base moved from sqlalchemy.ext.declarative to sqlalchemy.orm in 1.4 and was removed from the old location in 2.0.

Since SQLAlchemy has recommended use of inspect since 0.8, I've switched to it in wtforms_sqlalchemy.orm.

mapper.iterate_properties does not return relationships unless mappers have been fully initialized, which appears to be delayed in 2.0. However, accessing mapper.attrs causes initialization, so I've switched from mapper.iterate_properties to mapper.attrs.values.

There is a LegacyAPIWarning emitted due to the use of Query.get(), which is deprecated in 2.0 but not removed. Switching to the recommended select construct is not taken up here.

[jace]

Sorry for the merge commits. This will need a squash. Should I force-push one?

Also, does this need additional tests? I had to manually swap SQLAlchemy versions between test runs as it's not in the tox config. It may be worth adding (I'm not familiar with tox).

[azmeuk]

No need to force push, there is no conflict so this will be fine.
Now the only thing needed is that I find a bit of time to spend on reviewing this.

[azmeuk]

Thank you!
edit: I just released 0.4.0