SQLAlchemy has added support for computed col reflection; need to adapt Alembic tests

[zzzeek]

e.g.

Traceback (most recent call last):
File "/home/jenkins/workspace/alembic_master/00816f09/tests/test_autogen_computed.py", line 65, in test_remove_computed_column
is_(c.computed, None)
File "/home/jenkins/workspace/alembic_master/00816f09/.tox/py38-sqla13-sqlite-postgresql-mysql-oracle/lib/python3.8/site-packages/sqlalchemy/testing/assertions.py", line 264, in is_
assert a is b, msg or "%r is not %r" % (a, b)
AssertionError: Computed(<sqlalchemy.sql.elements.TextClause object at 0x7fb51dc02280>, persisted=False) is not None
assert Computed(<sqlalchemy.sql.elements.TextClause object at 0x7fb51dc02280>, persisted=False) is None

[zzzeek]

basically the tests and maybe the compare module need to be sensitive to the version of SQLAlchemy in use and adjust accordingly

[sqla-tester]

Mike Bayer has proposed a fix for this issue in the master branch:

Support computed reflection. https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/1797