Allow specification of Python executable

[jjtolton]

Currently the Python3 executable path is hardcoded to "python3". I propose we allow the executable to be specified via :python-executable in py/initialize!. On my system, Python3.6 always gets picked up first even though Python3.7 is available because Python3.6 is my current system alternative.

This change should also allow use of Python virtual environments.

[gigasquid]

Whether libpython-clj works with a virtual environment has come up a lot in general questions/ interactions. I think this will be very beneficial. 🎉

[orolle]

I try to get libpython-clj work with anaconda on windows. The windows PATH environment variable for clojure.java.shell/sh is handled a little bit different compared to Linux & Mac. Thus the hardcoded "python3" makes it impossible to execute anything with clojure.java.shell/sh. Being able to parameterize the python-executable would work around windows specifics.