Update requirements and pin black version

Refactor Travis config and add Coveralls

This can be removed after CI is in working order.

Accompanying this commit are a few test fixes (e.g. enable previously disabled
tests, remove tests with incorrect assumptions, skip tests earlier, mark more
tests as slow, etc.) and some minor refactoring.

Fix print statement in docs page

Use black formatting, update code style and conventions, miscellaneous pytest fixes

This commit contains necessary "Test"-prefix renamings.  Some test classes
were being automatically generated and `pytest` wouldn't pick them up unless
they matched the correct pattern.  Plus, the naming was all over the place, so
such changes were warranted anyway.

There were also some `pytest` constructor issues causing test classes to be
ignored.  In those cases, `setup_method` was used instead of `__init__`.

Closes #5.

Move from unittest/nose to pytest

Closes #6685.