distutils.spawn.find_executable to shutil.which

[Lestropie]

The MRtrix3 Python API depends on the distutils module for the find_executable() function. Apparently usage of this module is discouraged in favour of setuptools, and it is indeed intended for deprecation. But even then, with setuptools being to "download, build, install, upgrade, and uninstall Python packages", it's not necessarily a sensible dependency to have.

As of Python 3.3, the widely-used shutil module has the which() function, which serves the same purpose.

So if following through with #2047, I would suggest making the minimum dependency 3.3 (released Sep 2012), and making use of this function throughout.

[bjeurissen]

Makes sense!

[jdtournier]

I'd be happy enough this that too. It really should be OK to mandate Python 3.3 by now, but I expect we'll find it'll still cause friction in some cases...