Python version in docs

[aimichal]

The instructions for Python solvers say to use pip and python. I have two versions of Python installed (/usr/local/bin/python2.7 and /usr/local/bin/python3.5), and pip uses 2.7:

% head -1 /usr/local/bin/pip
#!/usr/local/opt/python/bin/python2.7

As currently written, the instructions didn't work because pip was installing into an old version of Pythong. To make it work, I had to do this:

• Instead of pip ..., I used python3.5 /usr/local/bin/pip ...
• Instead of python ..., I used python3.5 ...

I don't know how common this is, but if there's a good chance others will run into this situation, then the instructions should be written to explicitly specify Python 3.5.

[joelgrus]

most python ML people use virtualenv or conda to manage multiple python installations, and you should too!