Update Python autoformatting stack

[Zac-HD]

The autoformatting stack we use for Python is pretty good, but IMO shed is even better. I might be biased because I wrote it, but then again I wrote it specifically to be a more-comprehensive and easier-to-use version of Hypothesis' autoformatter stack.

Now that Hypothesis has dropped Python 3.5 support and we've migrated our CI, it makes sense to change over. I expect this to be a single PR, in four commits:

1. Run pyupgrade without the --keep-percent-format flag
2. Manually fix any issues that this created or %-formatting that it couldn't autofix
3. Fix any console blocks which don't start with >>> (git grep -A 3 '.. code-block:: pycon' finds them)
4. Convert over to run shed --refactor
5. Manually fix any issues or ugly formatting this created

Aiming to sort this out in early Feb after Zac-HD/shed#11 🙂