Allow passoing ExcludeConstraint to DropConstraint.from_constraint

Allow passoing ExcludeConstraint to DropConstraint.from_constraint

Added support for ``op.drop_constraint()`` to support PostrgreSQL
``ExcludeConstraint`` objects, as well as other constraint-like objects
that may be present in third party dialects, by resolving the ``type_``
parameter to be ``None`` for this case.   Autogenerate has also been
enhanced to exclude the ``type_`` parameter from rendering within this
command when  ``type_`` is ``None``.  Pull request courtesy David Hills.

Fixes: #1300
Closes: #1301
Pull-request: https://github.com/sqlalchemy/alembic/pull/1301
Pull-request-sha: b8643d19ab1930554e1137b64ffea8b8e2c74ec7

Change-Id: I25599d24a8bba455ab1d9b88843d8d84627a72d5