Bump matplotlib from 3.3.0 to 3.3.1 (AtsushiSakai#376)

Bumps [matplotlib](https://github.com/matplotlib/matplotlib) from 3.3.0 to 3.3.1.
- [Release notes](https://github.com/matplotlib/matplotlib/releases)
- [Commits](matplotlib/matplotlib@v3.3.0...v3.3.1)

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements.txt

@@ -1,5 +1,5 @@
 numpy == 1.19.1
 scipy == 1.5.2
 pandas == 1.1.0
-matplotlib == 3.3.0
+matplotlib == 3.3.1
 cvxpy == 1.1.4