Bump matplotlib from 3.5.3 to 3.6.1 in /extra_requirements

Bumps [matplotlib](https://github.com/matplotlib/matplotlib) from 3.5.3 to 3.6.1.
- [Release notes](https://github.com/matplotlib/matplotlib/releases)
- [Commits](matplotlib/matplotlib@v3.5.3...v3.6.1)

---
updated-dependencies:
- dependency-name: matplotlib
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

extra_requirements/requirements-tests.txt

@@ -6,7 +6,7 @@ galpy==1.8.1
 gitpython==3.1.30
 latex==0.7.0
 ipywidgets==8.0.4
-matplotlib==3.5.3; python_version<='3.7'
+matplotlib==3.6.1; python_version<='3.7'
 matplotlib==3.5.3; python_version>'3.7'
 mw_plot==0.9.0
 nbconvert==7.2.3