Vendor inspect.formatannotation
Vendor inspect.formatannotation

Vendored the ``inspect.formatannotation`` function inside of
``alembic.util.compat``, which is needed for the vendored version of
``inspect.formatargspec``.  The function is not documented in cPython and
is not guaranteed to be available in future Python versions.

Change-Id: Ia891131c0776f025cd12d359e03587819881eaee
Fixes: #657

zzzeek/alembic,sqlalchemy/alembic