Upgrade to CVXPY >= 1.1.13.

[bettbra]

A collection of mods while upgrading to CVXPY >= 1.1.13:

• Require python >= 3.6 (no harm since CVXPY 1.1 has that same requirement).
• Modify the nonconvex variables to take shape tuples, instead of the previous row, col ints. The new behavior is more consistent with CVXPY 1.
• Some minor PEP 8 formatting mods.
• Ensure all the tests and examples are working.
• Point to a new scsprox fork.

There's more to do to be sure, including sprucing up various docstrings. But that can wait for a separate pull request; seems more useful right now to restore the base to operation.

[SteveDiamond]

Thanks @bettbra this is a really great PR! I made some minor notes in my review. Generally looks good to go.

[SteveDiamond]

Did you run flake8 on it? That's what we use in cvxpy for linting. I would like to switch to black autoformatting, but that should be a separate PR.

[bettbra]

I haven't fixed all the flake8 issues yet but will later this evening. Gotta take the kids to football right now, ha!

[SteveDiamond]

Sounds good! Actually if it's easier you're welcome to just run "black *" to get all the formatting right, since I've reviewed the content. No need to fiddle with flake8 (unless you prefer it).

[SteveDiamond]

I decided to merge and apply black formatting myself, just for simplicity.