Allow specifying Python version when installing a package (i.e. --python flag)

[mforbes]

Reopening issue #3 (since there appears to be no way to change the owner).

Please provide a way of pinning the version of python used analogous to the pipx --python flag:

condax install --python=3.9 anaconda-project

(I recently ran into this on an M1 running Mac OS X where condax install anaconda-project pulled in python 3.11, breaking anaconda-project. Not quite sure why, but a --python flag would have provided a fix.)