[Backend Dependencies Update] Update alembic to 1.3.1

[pyup-bot]

This PR updates alembic from 1.2.1 to 1.3.1.

Changelog

1.3.1

:released: November 13, 2019

 .. change::
     :tags: bug, mssql
     :tickets: 621

     Fixed bug in MSSQL dialect where the drop constraint execution steps used
     to remove server default or implicit foreign key constraint failed to take
     into account the schema name of the target table.


.. changelog::

1.3.0

:released: October 31, 2019

 .. change::
     :tags: feature, command
     :tickets: 608

     Added support for ALEMBIC_CONFIG environment variable,
     refers to the location of the alembic configuration script
     in lieu of using the -c command line option.


 .. change::
     :tags: bug, autogenerate
     :tickets: 131

     Fixed bug in new Variant autogenerate where the order of the arguments to
     Variant were mistakenly reversed.

 .. change::
     :tags: change, compatibility

     Some internal modifications have been made to how the names of indexes and
     unique constraints work to make use of new functions added in SQLAlchemy
     1.4, so that SQLAlchemy has more flexibility over how naming conventions
     may be applied to these objects.

.. changelog::

Links

• PyPI: https://pypi.org/project/alembic
• Changelog: https://pyup.io/changelogs/alembic/
• Homepage: https://alembic.sqlalchemy.org
• Docs: https://pythonhosted.org/alembic/

[pyup-bot]

Closing this in favor of #778