[IMP-dev] IMP CMake builds now default to Python3

Note that in IMP's develop branch (as of 9f63080a6, committed earlier today) CMake will now default to using Python 3 if available: https://github.com/salilab/imp/commit/9f63080a6

This new logic should make building IMP on systems with multiple Pythons (e.g. Macs with Homebrew) much simpler, as CMake should handle this without needing to manually set PYTHON_INCLUDE_PATH or PYTHON_LIBRARY directly.

If you still want to build IMP with Python 2 on a system that has Python 3, pass -DUSE_PYTHON2=on to your cmake invocation. (But note that Python 2 is - very slowly - being phased out so switching to 3 might make sense. For example Fedora 30 no longer has Python 2 scikit-learn or biopython packages.)

Ben