Rewrite unit tests to use Python unittest

[mbrukman]

This lets us drop the dependency on the deprecated nose library and use the
Python standard library unittest. Since we don't use any advanced
functionality, and the unittest library is sufficient for our use cases, this
means we can remove the dependency on nose without introducing a new
dependency on either nose2 or pytest.

Closes #61

[mbrukman]

All tests are passing! However, we're still running the tests using nosetests — I wanted to change as little as possible, to make sure everything is still working.

Now, we can remove our dependency on nose and run tests via python -m unittest unstead of nosetests and get the same functionality. Hopefully, it'll be even slightly faster overall due to having to install fewer packages!

[mbrukman]

Looks like all tests are "passing", but Python 2 isn't actually running tests. Maybe it needs python -m unittest discover rather than simply python -m unittest which seems to work in Python 3?

[mbrukman]

Our tests are now faster (this is Python 3.7 on Ubuntu 20.04, sample size of 1, so usual caveats):

• Before, via nosetests: "Ran 10 tests in 0.742s"
• After, via unittest: "Ran 10 tests in 0.008s"

And now, python -m unittest discover works in Python 2 and Python 3, and dropping the nose dependency lets us enable running on Python 3.10 as well with no changes!

Success! 🎉