Bump matplotlib from 3.4.0 to 3.4.1 (AtsushiSakai#497)

Bumps [matplotlib](https://github.com/matplotlib/matplotlib) from 3.4.0 to 3.4.1.
- [Release notes](https://github.com/matplotlib/matplotlib/releases)
- [Commits](matplotlib/matplotlib@v3.4.0...v3.4.1)

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 == 1.20.2
 scipy == 1.6.2
 pandas == 1.2.3
-matplotlib == 3.4.0
+matplotlib == 3.4.1
 cvxpy == 1.1.11