Bump gitpython from 3.1.27 to 3.1.28 in /extra_requirements

Bumps [gitpython](https://github.com/gitpython-developers/GitPython) from 3.1.27 to 3.1.28.
- [Release notes](https://github.com/gitpython-developers/GitPython/releases)
- [Changelog](https://github.com/gitpython-developers/GitPython/blob/main/CHANGES)
- [Commits](gitpython-developers/GitPython@3.1.27...3.1.28)

---
updated-dependencies:
- dependency-name: gitpython
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

extra_requirements/requirements-tests.txt

@@ -3,7 +3,7 @@ astropy==5.1; python_version>'3.7'
 coverage==6.5.0
 coveralls==3.3.1
 galpy==1.8.0
-gitpython==3.1.27
+gitpython==3.1.28
 latex==0.7.0
 ipywidgets==8.0.2
 matplotlib==3.5.3; python_version<='3.7'