Update gitpython to 3.1.0

[pyup-bot]

This PR updates GitPython from 3.0.7 to 3.1.0.

Changelog

3.1.0

=====

* Switched back to using gitdb package as requirement
(`gitdb59 <https://github.com/gitpython-developers/gitdb/issues/59>`_)

3.0.9

=====

* Restricted GitDB (gitdb2) version requirement to < 4
* Removed old nose library from test requirements

Bugfixes
--------

* Changed to use UTF-8 instead of default encoding when getting information about a symbolic reference
(`774 <https://github.com/gitpython-developers/GitPython/issues/774>`_)
* Fixed decoding of tag object message so as to replace invalid bytes
(`943 <https://github.com/gitpython-developers/GitPython/issues/943>`_)

3.0.8

=====

* Added support for Python 3.8
* Bumped GitDB (gitdb2) version requirement to > 3

Bugfixes
--------

* Fixed Repo.__repr__ when subclassed 
(`968 <https://github.com/gitpython-developers/GitPython/pull/968>`_)
* Removed compatibility shims for Python < 3.4 and old mock library
* Replaced usage of deprecated unittest aliases and Logger.warn
* Removed old, no longer used assert methods
* Replaced usage of nose assert methods with unittest

Links

• PyPI: https://pypi.org/project/gitpython
• Changelog: https://pyup.io/changelogs/gitpython/
• Repo: https://github.com/gitpython-developers/GitPython
• Docs: https://pythonhosted.org/GitPython/