By default, use pip for fetching and installing python packages.

[jordansissel]

This adds a new flag, --python-internal-pip, which is enabled by default.

"internal pip" means using 'python -m pip' to invoke pip. Ideally this will make fpm more correctly use pip.

Tested on python 2.7.17 and 3.6.9 on Ubuntu 18.04

All python tests passing 👍👍

Fixes #1820

[jordansissel]

A cool thing is that some python tests were failing before and this helped fix things. One of the tests was using python3 and specifying "pip" which, on my workstation, was python2's pip. This made for amusing clashes as pip downloaded an ancient version of Django and python3 failed trying to install it.

[jordansissel]

pip now complains about being invoked.

WARNING: pip is being invoked by an old script wrapper. This will fail in a future version of pip.
Please see pypa/pip#5599 for advice on fixing the underlying issue.
To avoid this problem you can invoke Python with '-m pip' instead of running pip directly.

[jordansissel]

% bundle exec rspec spec/fpm/package/python_spec.rb
.............

Finished in 12.2 seconds (files took 0.22577 seconds to load)
13 examples, 0 failures

With the last commit, the test suite now passes. 👍

[jordansissel]

% bundle exec bin/fpm -s python -t deb --python-bin python3 django
Debian packaging tools generally labels all files in /etc as config files, as mandated by policy, so fpm defaults to this behavior for deb packages. You can disable this default behavior with --deb-no-default-config-files flag {:level=>:warn}
Created package {:path=>"python-django_3.2.9_all.deb"}

Django seems to be packaging ok.

[jordansissel]

I'm nervous about merging this, but I gain confidence with passing rspec tests as well as manual tests.

Two manual tests:

Running using the new default, internal pip aka "python -m pip":

% bundle exec bin/fpm -s python -t deb --python-bin python3 requests
Created package {:path=>"python-requests_2.26.0_all.deb"}

Running with internal pip disabled. This should fall back to easy_install by default, which isn't present on my system:

% bundle exec bin/fpm -s python -t deb --python-bin python3 --no-python-internal-pip requests
Need executable 'easy_install' to convert python to deb {:level=>:error}