Bump matplotlib from 3.4.2 to 3.4.3 (#539)

Bumps [matplotlib](https://github.com/matplotlib/matplotlib) from 3.4.2 to 3.4.3.
- [Release notes](https://github.com/matplotlib/matplotlib/releases)
- [Commits](https://github.com/matplotlib/matplotlib/compare/v3.4.2...v3.4.3)

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

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requirements.txt

@@ -1,5 +1,5 @@
 numpy == 1.21.2
 scipy == 1.7.1
 pandas == 1.3.1
-matplotlib == 3.4.2
+matplotlib == 3.4.3
 cvxpy == 1.1.15