Update development dependencies for p4python, Swift, and S3.
Review Request #11357 — Created Dec. 30, 2020 and submitted
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.
pip install -r dev-requirements.txtfor each version of Python.
Verified the expected packages were installed.