v21.8.4

This is a minor bug fix release primarily for Windows installation.
There is no feature change.

Installation:

• Application installation on Windows works again. This fixes #2610
• We now build and test app bundles on all supported Python versions: 3.6 to 3.9

Thank you to @gunaztar for reporting the #2610 bug

Documentation:

• Documentation is updated to reference supported Python versions 3.6 to 3.9