Consider switching to py.test

[audiodude]

Nose (nosetests), which we use as our test harness/test runner, has the following note on its docs page:

Note to Users
Nose has been in maintenance mode for the past several years and will likely cease without a new person/team to take over maintainership. New projects should consider using Nose2, py.test, or just plain unittest/unittest2.

We should switch to py.test. It shouldn't be so hard, since we're not using any nose specific features besides automatic test collection, which I believe py.test has as well.