Don't use type(None) as fallback for Computed or Identity

[CaselIT]

In old version of sqlalchemy, that don't have Computed or Identity a fallback in alembic is used.
The current fallback is to set these values to type(None)

This causes issues since checks like isinstance(something, Computed) will pass when something is None, leading to attribute errors.

[hustliyilin]

One of the examples is as the following:

File "/usr/local/lib/python3.6/site-packages/alembic/ddl/sqlite.py", line 56, in requires_recreate_in_batch
    and col.server_default.persisted
AttributeError: 'NoneType' object has no attribute 'persisted'

[sqla-tester]

Federico Caselli has proposed a fix for this issue in the main branch:

Fix compat issue with older SQLAlchemy versions. https://gerrit.sqlalchemy.org/c/sqlalchemy/alembic/+/4601

[zzzeek]

looks like we added "Computed" in 1.3.11