Bump matplotlib from 3.3.4 to 3.4.0 (AtsushiSakai#492)

Bumps [matplotlib](https://github.com/matplotlib/matplotlib) from 3.3.4 to 3.4.0.
- [Release notes](https://github.com/matplotlib/matplotlib/releases)
- [Commits](matplotlib/matplotlib@v3.3.4...v3.4.0)

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 == 1.20.1
 scipy == 1.6.2
 pandas == 1.2.2
-matplotlib == 3.3.4
+matplotlib == 3.4.0
 cvxpy == 1.1.11