Issue 180: Fix not parsing pip.conf index urls

[calizarr]

Fixes: #180

pip prefers the usage of the CLI tool to get information, therefore I resorted to subprocess for this. This should get the information from the pip CLI tool and then add it into the index_urls tuple.

Thanks.

[calizarr]

I cannot get a good grasp on why the tests are failing. The diffs are a bit too long and the only change should be in the index_urls. If empty, then it should be equivalent to the same as before. If not, then yes things would change, but searching for the specific part in the diff is almost impossible.

[calizarr]

After running:

./configure --clean
./configure --dev
PYINSP_REGEN_TEST_FIXTURES=yes ./venv/bin/python -m pytest -vvs --color=yes

Then there are some pycodestyle failures in my recently added code, however, there are still other failures that have to do with package versions not being exact. I can change those etc. although it's certainly not what this PR is meant to do.

[sschuberth]

Note that @heliocastro already filed #170, which seems to be related.

[calizarr]

Note that @heliocastro already filed #170, which seems to be related.

They are incredibly related if not almost identical. However, pip warns against using it as a library

As noted previously, pip is a command line program. While it is implemented in Python, and so is available from your Python code via import pip, you must not use pip’s internal APIs in this way.