Standardize versions of Python and PyQt5 used on all platforms

[jg18]

The versions used for Python and package dependencies should be consistent across platforms, such as between Pipfile and the macOS build scripts.

My recommendations:

• Python 3.8.6 (Python 3.9.0 is too new, given that it's the first release of 3.9)
• PyQt5 5.11.3 (the last version to include QtWeb* components on macOS)

[jg18]

@chief1983 can you help with this?

The Pipfile may need other package versions updated for compatibility with PyQt5 5.11.3. Manually attempting to build on Windows with PyQt5==5.11.3 might indicate which packages need to be limited to specific versions.