Add pip.conf heads-up to PyPI.org migration guide

[brainwane]

Followup to https://github.com/pypa/packaging-problems/issues/135 :

A user had a .pip directory containing a pip.conf file that pointed to the old PyPI URL:

[global]
index-url=https://pypi.python.org/pypi/

I think https://packaging.python.org/guides/migrating-to-pypi-org/#downloading-packages should include a heads-up about that. Once you figure out the language to add, and add it, we'll revise the https://pyfound.blogspot.com/2018/03/warehouse-all-new-pypi-is-now-in-beta.html#Migrating section to point to it.

[pgadige]

I'd like to work on this. I've scribbled few lines to try to summarize the essence of pip.conf file.

Besides checking contents of ~/.pypirc and result of $PIP_INDEX_URL, ensure you have a file pip.conf, which contains a link to the default host for downloading packages, in the directory .pip/. The absence of the file may cause an error while downloading packages.

Please pardon me if I've misinterpreted the context of pip.conf in the aforementioned issue.

@brainwane, could you please review the above text and suggest edits? Please let me know if I've missed out anything else.

Thank you.

[brainwane]

@pradyunsg @jonparrott could you help with this? I don't know enough about pip.conf files to advise. :) Thanks!

[brainwane]

@kennethreitz @dstufft could you check that text for accuracy? Thanks!

[pradyunsg]

I completely missed my mention earlier. :/

The following link https://pip.pypa.io/en/stable/user_guide/#configuration contains all the information about pip.conf.

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The text seems fine to me. I'd suggest not using $PIP_INDEX_URL (since that's Unix specific) - "PIP_INDEX_URL environment variable" should be a good replacement.