Overhaul testing for better coverage reporting

[kurtmckee]

This introduces the following changes:

• All CPython versions are tested with and without the C extension (locally).
• Coverage is tracked for all Python interpreter versions, and is then combined to give a total report. A report is output during tox execution, and an HTML report is written to htmlcov/.
• Travis CI and coveralls are removed, as they are unused in GitHub CI and locally.
• CPython 3.8 is tested in CI now.
• Tox environment dependencies are now set, so the test suite can be executed in parallel using tox run-parallel or simply tox p.

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Coverage report screenshot

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After reviewing the coverage report, there are some improvement opportunities in the test suite. For example, this paradigm is used several places:

try:
    cause_a_crash()
    assert False
except ValueError as e:
    assert e.something == "expected"

This can be replaced with pytest constructs:

with pytest.raises(ValueError) as e:
    cause_a_crash()
assert e.something == "expected"

This can be done in subsequent work.