fix test_find_cmd_python

It's a bit unclear what should be done here.
'python' is short-circuited in find_cmd to sys.executable.
This commit does the same short-circuit with basename(sys.executable), which can be any of Python, python, python3, python3.exe, etc.

An alternate fix would be to change the test back to testing what the code actually did.
remove special case for 'python' in find_cmd
Merge pull request #3342 from minrk/executable

fix test_find_cmd_python