Remove the Pants Python interpreter from PATH.

[jsirois]

Since this interpreter is hobbled - it cannot build C extensions - it is
dangerous to have it discoverable on the PATH. Anyone needing to build C
extensions needs to supply their own local Python interpreter.

Fixes #69

[jsirois]

I'll follow up with a 0.2.2 release.

And the prep is now up in #72.

[jsirois]

N.B.: Although this solves #69 it does not solve #71. #71 requires some hard decisions / restructure how 3rdparty plugins are packaged or consumed or both or it requires outlawing dependencies on distributions without wheels for the supported Pants platforms out on PyPI.

[sureshjoshi]

Is this something we'd want to mark in the getting started documentation, that if they need to build C extensions then (...)?

Wait, no, nevermind. This runs our stuff, not user stuff. That should be done using their selected Python interpreter

[jsirois]

Yeah, exactly. No different than with ./pants since we never added that venv interpreter to the PATH.