python-r1.eclass: Support python_check_deps() in python_setup

python-r1.eclass: Support python_check_deps() in python_setup

Provide an alternate mode for python_setup() that behaves similarly to
python-any-r1 eclass. If python_check_deps() function is declared
by the ebuild, the python_setup logic switches to accepting any
implementation that is in PYTHON_COMPAT, installed and satisfies
python_check_deps() independently of USE flags.

This new logic makes it possible to replace some of the existing
REQUIRED_USE constraints for build-time dependencies with more friendly
any-of dependencies. For example, if a package supports both Python 2 &
Python 3 but has a purely Python 2 build-time dependency (e.g. for
building documentation) we had to force Python 2 being enabled via
REQUIRED_USE. Using python_check_deps() with appropriate any-of
dependency, we can use Python 2 for this task without actually forcing
the user to change USE flags or install the package for Python 2.