This should increase coverage for subprocesses, where previously
`source` paths were used only from the config file, but not the initial
`--source` argument.
- Skips pypy for coverage, reports only py37 to coveralls
- tox: allow for TOXENV=py37-coverage
- tracks coverage in subprocesses, using coverage-enable-subprocess, and
parallel=1
- removes usedevelop with doctesting to match `--source` being used with
coverage
- keep coveralls for now, used with AppVeyor
Daniel Hahler
hugovk
Tyler Goodlet
Bruno Oliveira