Replace doctests with unit tests

[jace]

Doctests make documentation unnecessarily verbose, and are often incompatible between Python 2 and 3 because of differences in repr. They also can't be run in isolation to test one thing at a time. All doctests should be replaced with unit tests. Replacement is easy as each line can be reproduced as an assertEqual.

[jace]

The problems with doctests are no longer relevant since the switch to pytest. However, the case remains that they make documentation ugly. pytest also makes it easier to write tests, so we should just switch anyway.

[jace]

This is now the subject of #237.