Generate python 3.9 wheel files

[rgil-splunk]

• Generate wheels using python 3.9 as well as python 3.6.
• If there are wheel differences, add 3.9 wheels to the pip39_dependencies key.
• Modularize the pip dependency key checks to easily allow support for new python versions in the future.
• Always generate wheels under respective directory (shared, py3, etc.) without flattening for simplicity.

Tested using:

• Unit tests (which were accordingly updated)
• Manually installed an app and ran tests on an OVA using python 3.9