Update development dependencies for p4python, Swift, and S3.
Update development dependencies for p4python, Swift, and S3.

When setting up a development environment, we now use the latest version
of p4python (which added Python 3.8 support, but does not include 3.9),
and our Swift and S3 packages, which are needed for unit tests.

Testing Done:
Ran `pip install -r dev-requirements.txt` for each version of Python.
Verified the expected packages were installed.

Reviewed at https://reviews.reviewboard.org/r/11357/

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