Allow passing --allow-external to pip about pipsi

[bitprophet]

FWIW I found that it's possible to work around this by supplying shell environment variables to the invoking environment; pipsi's use of subprocess preserves the env well enough that you can, for example, do PIP_ALLOW_EXTERNAL=yes pipsi install foobar, as per https://pip.pypa.io/en/stable/user_guide/#environment-variables .

(In my case I didn't literally try allow-external, but I did have success using PIP_TIMEOUT=60 PIP_PROCESS_DEPENDENCY_LINKS=yes pipsi install path/to/private/git/url.)

[kevinastone]

👍

ReviewBoard Tools fails as well:

~> pipsi install RBTools
Using real prefix '/System/Library/Frameworks/Python.framework/Versions/2.7'
New python executable in $HOME/.local/venvs/rbtools/bin/python
Installing setuptools, pip...done.
Downloading/unpacking RBTools
  Could not find any downloads that satisfy the requirement RBTools
  Some externally hosted files were ignored (use --allow-external RBTools to allow).
Cleaning up...
No distributions at all found for RBTools
Storing debug log for failure in $HOME/.pip/pip.log
Failed to pip install.  Aborting.

[RonnyPfannschmidt]

this needs some kind of configuration to remember it for recreation