Bump gitpython from 2.1.11 to 2.1.12

Bumps [gitpython](https://github.com/gitpython-developers/GitPython) from 2.1.11 to 2.1.12.
- [Release notes](https://github.com/gitpython-developers/GitPython/releases)
- [Changelog](https://github.com/gitpython-developers/GitPython/blob/master/CHANGES)
- [Commits](gitpython-developers/GitPython@2.1.11...2.1.12)

Signed-off-by: dependabot-preview[bot] <[email protected]>

Author: dependabot-preview[bot]

requirements.txt

@@ -1,7 +1,7 @@
 tornado==5.1.1
 requests==2.20.1
 pyjwt==1.7.1
-gitpython==2.1.11
+gitpython==2.1.12
 there==0.0.9
 mock==2.0.0
 cryptography==2.3.1