Update matplotlib requirement from ~=3.3.0 to ~=3.3.1 (#625)

Updates the requirements on [matplotlib](https://github.com/matplotlib/matplotlib) to permit the latest version.
- [Release notes](https://github.com/matplotlib/matplotlib/releases)
- [Commits](matplotlib/matplotlib@v3.3.0...v3.3.1)

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

Co-authored-by: dependabot-preview[bot] <27856297+dependabot-preview[bot]@users.noreply.github.com>

Author: dependabot-preview[bot]

requirements-dev.txt

@@ -5,7 +5,7 @@ pytest ~=6.0.1
 vcrpy ~=4.1.0
 doctr ~=1.8.0
 ipython ~=7.17.0
-matplotlib ~=3.3.0
+matplotlib ~=3.3.1
 numpydoc ~=1.1.0
 requests-mock ~=1.8.0
 sphinx ~=3.1.2