Docs - replace python with python3 for intersphinx

[stevepiercy]

Our intersphinx links and many other links to Python docs point to version 2.x, e.g.,

:ref:`Python Packages <python:tut-packages>`

We should be using this:

:ref:`Python Packages <python3:tut-packages>`

...where we edit docs/conf.py to generate intersphinx links to the Python 3.x docs concurrently with Python 2:

    'python': (
        'http://docs.python.org/2',
        None),
    'python3': (
        'https://docs.python.org/3/',
        None),

Some links might change between Python 2 and 3, so each rendered link needs to be tested.

Assuming all links to Python 2 can be changed to 3, then docs/conf.py can drop the intersphinx entry for 'python' as the final step of this task.

I'm already doing this for the branch docs/easy-install-to-pip.2104 and issue #2223, but there are other docs that are outside the scope of that branch/issue that will also need review.

[mmerickel]

I would just re-alias python to python3's url and add a python2 if anything needs to ref py2 explicitly.

[stevepiercy]

When I did this:

 'python': (
        'https://docs.python.org/3',
        None),

and

:ref:`Python Packages <python:tut-packages>`

Links still pointed at Python 2 docs. I even blasted away the build directory. Do you mean something else?

[mmerickel]

That seems like something is still sticking around for whatever reason.

[mmerickel]

@stevepiercy don't start tagging this pip work on the 1.7 milestone unless you're positive it's getting done in the next few weeks.

[stevepiercy]

Unless something untoward should happen to me, it will.

[stevepiercy]

I couldn't figure out why things stuck around. Were you able to replicate the issue? Could it be some obscure default in intersphinx?

[mmerickel]

I'm pretty anti changing every reference in the doc to python3: so if we can't figure this out then I opt to keeping things at python2 until we can. Version-specific directives are a pain to maintain.

[stevepiercy]

Were you able to replicate the issue? I'd like to know if it's just my setup. If you can replicate it, then I'll try a clean virtualenv to eliminate any possible stickiness.

[stevepiercy]

It was me. There's a stray docs/quick_tutorial/conf.py and I thought I was editing docs/conf.py. argh.

[stevepiercy]

Closed by #2431