Accommodate SQLAlchemy 1.4/2.0

Accommodate SQLAlchemy 1.4/2.0

To accommodate SQLAlchemy 1.4 and 2.0, the migration model now no longer
assumes that the SQLAlchemy Connection will autocommit an individual
operation.   This essentially means that for databases that use
non-transactional DDL (pysqlite current driver behavior, MySQL), there is
still a BEGIN/COMMIT block that will surround each individual migration.
Databases that support transactional DDL should continue to have the
same flow, either per migration or per-entire run, depending on the
value of the :paramref:`.Environment.configure.transaction_per_migration`
flag.

Compatibility is established such that the entire library should
not generate any SQLAlchemy 2.0 deprecation warnings and
SQLALCHEMY_WARN_20 is part of conftest.py. (one warning remains
for the moment that needs to be resolved on the SQLAlchemy side)

The test suite requires SQLAlchemy 1.4.0b2 for testing 1.4;
1.4.0b1 won't work.

Test suite / setup also being modernized, as we are at
SQLAlchemy 1.3 we can now remove the majority of the testing
suite plugin.

Change-Id: If55b1ea3c12ead66405ab3fadc76d15d89dabb90