Bump pandas version in development dependencies for Python 3.9 support

[replomancer]

Description

This PR adds Python 3.9 to github workflows and setup.py, and also updates pandas version in pyproject.toml so that the build process in Python 3.9 can succeed. Closes #343

How has this been tested?

• The build process has been manually tested in a freshly created Python 3.9 environment and automated tests were run with make test [1].
• The build process and automated tests have also been run in a Python 3.8 environment (pandas version has been bumped for both 3.8 and 3.9).

[1] tests/notebooks/test_all_notebooks.py is currently failing because jupyter doesn't support Python 3.9 yet (e.g. see: https://pyreadiness.org/3.9/ , https://jupyter-client.readthedocs.io/en/stable/changelog.html).

Checklist

• [x] I have followed the Contribution Guidelines and Code of Conduct
• [ ] I have commented my code following the OpenMined Styleguide
• [x] I have labeled this PR with the relevant Type labels
• [ ] My changes are covered by tests