Unify the adding of libs into the path in one place

[vojtechjelinek]

Currently, we add the third party libs directories to sys.path in multiple files:

• appengine_config.py
• core/tests/gae_suite.py
• scripts/linters/pre_commit_linter.py
• scripts/run_backend_tests.py

unify this into one place so the updates to libraries are more easy to do.

[tanishkab]

Please assign this issue to me. Thank You in advance

[vojtechjelinek]

@Tanishka-Bansal Thanks for the interest, please go through these instructions first https://github.com/oppia/oppia/wiki/Contributing-code-to-Oppia and then you can try some issue with good first issue label.

[oppiabot]

Hi @oppia/core-maintainers, this issue is not assigned to any project. Can you please update the same? Thanks!

[U8NWXD]

We no longer need to change the PATH so much now that we have #15602, so I don't think this is relevant anymore