We currently have `pytest` with the coverage plugin running over our tests in a `tests` directory.

What's the simplest way to also run doctests extracted from our main code? `--doctest-modules` doesn't work (probably since it just runs doctests from `tests`). Note that we want to include doctests in the same process (and not simply run a separate invocation of `py.test`) because we want to account for doctest in code coverage.