test: make doctests work

[jsstevenson]

https://docs.python.org/3/library/doctest.html

Kind of useful to ensure that our examples stay up to date as code changes. But also, very slow to run, since our examples re-initialize resources ever time (which I'd prefer to keep, it makes examples self-sufficient, w/o having to scroll up to see how objects were created). The old default setup.cfg file that we inherited from Alex included running module doctests as part of the default pytest args but I think that's an unnecessary practice given this -- main tests are for checking for regressions and ensuring functions, and you should only run doctests once an issue is ~complete to ensure that examples stay current.

• Note that doctests perform basic string-to-string comparisons of REPL output. It's a little inelegant.
• async support kinda sucks. It presumes a basic Python REPL context, which means you can't await anything outside of a function. Worse, using multiple async.run calls in a single docstring causes some sort of problem (either unfixable w/r/t doctests themselves, or something about how we're using asyncpg), so I'm currently not sure how to show multiple discrete examples for async functions.

[korikuzma]

Suggested change

	Note that most methods are defined as Python coroutines, so the must be called
	Note that most methods are defined as Python coroutines, so they must be called