Delete requirements.txt; related CI & test changes

[kandersolar]

Description

There are two main reasons we might want to have a requirements.txt:

• As an environment spec for users. But pvanalytics is a library, not an application, so we prefer dependency ranges in setup.py over dependency versions in a requirements file.
• As an environment spec for testing, if you need specific versions beyond what's specified in setup.py. We don't (currently) need that, so there's no need for a requirements.txt file either. The exception is requirements-min.txt, which of course is not equivalent to setup.py.

This PR deletes requirements.txt so there's one less place to keep track of dependencies. This required a bit of reworking of the CI configuration, which then uncovered some minor oversights in the tests.

Checklist

• Closes Requirements in setup.py are redundant, and partially inconsistent, with requirements.txt #113
• Pull request is nearly complete and ready for detailed review