Bump matplotlib from 3.1.2 to 3.4.1

Bumps [matplotlib](https://github.com/matplotlib/matplotlib) from 3.1.2 to 3.4.1.
- [Release notes](https://github.com/matplotlib/matplotlib/releases)
- [Commits](matplotlib/matplotlib@v3.1.2...v3.4.1)

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

Author: dependabot-preview[bot]

requirements.txt

@@ -3,7 +3,7 @@ xgboost==1.0.0
 seaborn==0.10.0
 scipy==1.4.1
 pandas==1.0.1
-matplotlib==3.1.2
+matplotlib==3.4.1
 numpy==1.18.1
 astroquery==0.4.1
 scikit_learn==0.22.1