use python version define in PY_BIN if it exists

[hoexter]

Check if we've an executable file provided in an env var called
PY_BIN. If that's the case make it the first choice for the
validation of the python version requirements.
As a fallback populate the list of python options to try with
all python and python3 versions we can find in the PATH
of the environment.

[hoexter]

Hi @fboender,
this is my somewhat naive approach to tackle the issue described in #189. Variable names are kind of arbitrarily chosen.