Drop support for Python 2.7

[rdb]

Python 2.7 is officially already EOL since January 1st. Continuing to support it is a big burden on Panda3D maintenance. Python 3 has been available for a very long time, is quite stable in Panda3D, and we have given ample warning about the imminent removal of Python 2.7. I think it is about time we stopped needing to give any thought to Python 2.

The final release of the 2.7 branch, 2.7.18, occurs on April 18th. I suggest that after that point:

• We remove Python 2.7 from supported platforms in setup.cfg.
• The buildbot stops building Python 2.7 builds from the master branch of Panda3D
• We remove Python 2.7 from CI tests
• makewheel explicitly refuses to build wheels for Python 2.
• We remove deploy-ng support for Python 2 (Remove python2 code from deployment tools #889, Python2 code remove from deploy_stub  #892).
• We clean up all the places where we have to bend over backwards to support Python 2.
• We update sample code to use Python 3 idioms where Python 2-isms are used
• In the 1.10 manual, we put warnings in all places relevant to Python 2.
• In the 1.11 manual, we remove anything specific to Python 2 and indicate that we do not support it.

We can continue building the 1.10 releases for now against Python 2.7.18, but if that becomes too much effort, I would be open to stopping this as well.