Limited support for Python 3.5

[mmaney]

@mmaney has been trying out github's actions for running lint & tests against multiple Python versions, and there appear to be some issues with sewer and 3.5. This bug will serve to record what I find.

Python features (ssen in non-test sewer code)

• f-strings, of course. the GH checks have found a number of them, and I'm rewriting them as I become aware of them.
• comma after kwargs - this was caused by having the (obsolete) --py36 option for black, I think. That's been removed from sewer's setup & docs
• underlines in numeric literals, pfui

Secondary issues

• 3.5 doesn't accept type hints on variables. Workaround is to use hints in comments (or sometimes just to skip them)
• black isn't compatible with 3.5, period. Workaround for the GH actions is to skip that as a requirement.