This change includes a few things:
1) Prefer python2 and pip2 when on the PATH, over the undecorated
names python and pip. This is the standard convention for package
managers like Pip, etc., to support Python2 and Python3 side-by-side.
2) Fail-fast if neither can be found on the PATH.
3) Check the reported version number for python, pip, and pylint, and
fail-fast if it doesn't report back 2.7, just to safeguard against
undecorated binaries with unsupported versions.
Also, we had not listed wheel as a dependency in the requirements.txt
file. This needs to be there to support building bdist_wheels. Fixed.
440ffb27e5