Install pypy3.5-7.0.0 and make it available

[humitos]

I think it's a good idea to start having a pypy version in our docker images.

David found that this could speed up many builds. So, it could be a suggested way for "My builds are taking too much time" in the FAQ section once this is tested a little more.

Reference: readthedocs/readthedocs.org#3870

[stsewd]

I'm curious how we execute commands here? We have a python binary too?

[humitos]

Yes! Same flow as usual should work:

~ ⌚ 23:47:44
$ pyenv shell pypy3.5-7.0.0 

~ ⌚ 23:47:52
$ python
Python 3.5.3 (928a4f70d3de7d17449456946154c5da6e600162, Feb 09 2019, 11:50:43)
[PyPy 7.0.0 with GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>>> 

[ericholscher]

Makes sense to me. Sad to make things larger. I do wonder if we should start splitting up docker images based on Python version, but that just creates a different problem and more total space :/

[humitos]

I'm merging this one.

We will need to adapt our YAML code to be able to allow something like pypy3.5-7.0.0 or pypy3.5 or similar.

[bebehei]

At least this typo broke the current master.

[humitos]

Thanks. Fixed at #96