Translate pull_requests.sh into Python and make it part of brainmatch.py

[eurunuela]

Summary

We should make the entire package Python-based for better and easier maintenance.

Additional details

This issue stems from a discussion in issue #32 of brainhackorg/global2021.

Next steps

1. Create a method that replicates the GitHub CLI calls from Python using subprocess.
2. Call this method in brainmatch.py before the matching process.

[eurunuela]

@jhlegarreta could you assign the issue to me? It will help me track it on my notifications page. Thanks!