Migrate to pytest

[pschanely]

Pytest seems like it might be better for us then unittest:

• it works better with mypy (because mypy understands real asserts)
• It's easier to selectively run tests (CrossHair has a lot of not-fast tests)

[pschanely]

Many tests are in pytest now, and we're preferring pytest for new tests. pytest also runs unittest tests, so we're not (yet) aggressively converting old tests.

[pschanely]

We have a lot of tests still using unittest. If you're looking for a straightforward way to get involved in CrossHair, converting some *_test.py files to pytest might be a good start!

[pschanely]

There are only about 10 test files left using unittest, and I think they are all simple (no setup, teardown, or subTests). We are close!