Run the doctests with pytest

[asmeurer]

Right now the doctests are run with a separate script, ./run_doctests. This is done because we have to patch the doctest runner to get the behavior we want.

It would be nice to get it so that pytest runs the doctests. Possible ways to do this:

1. Hook into the pytest runner in run_doctests so that the collecter sees them as tests and automatically runs them. I don't know how hard this is to do.
2. Upstream the improvements to pytest itself. See Add an ini option doctest_standalone_namespace pytest-dev/pytest#6978 and Doctest should ignore lines starting with ``` when run on Markdown files pytest-dev/pytest#7374. This is the preferred option, but the challenge is getting pytest to accept the changes.
3. Make a custom test that runs run_doctest. The issue here is how to make it show the output nicely, especially when there are failures. I don't know if there is a way to do it, other than to basically do option 1.