This change makes it so that python3 is required by default to build
MicroPython. Python 2 can be used by specifying make PYTHON=python2.

This comes about due to a recent-ish change to PEP 394 that makes the
python command more optional than before (even with Python 2 installed);
see https://github.com/python/peps/commit/cd59ec03c8ff1e75089d5872520cd0706774b35b#diff-1d22f7bd72cbc900670f058b1107d426

Since the command python is no longer required to be provided by a
distribution we need to use either python2 or python3 as commands.  And
python3 seems the obvious choice.