Push /usr/bin/python to almost the bottom of the barrel. This makes the strategy to prefer
specific versions of the "mystery meat" version.
* Change INTERPRETER_PYTHON default to 'auto'
Change description to match change in behavior.
Change deprecation message to a warning.
* Update docs
* Add porting guide entry
* Update unit tests
* Update integration test
* Allow INTERPRETER_PYTHON_FALLBACK to be configure using a variable
* Prefer platform-python above other Python 2 interpreters
* Add Python 3.10 to the list of interpreters
