Speed up coverage reporting by skipping package installation

[kurtmckee]

Also, modify the Makefile to use tox for coverage reporting.
Locally, make coverage runs in under 7 seconds.

If make coverage is not fast enough, I can remove the Makefile change.

Makefile change removed.

[coveralls]

coverage: 60.998%. remained the same
when pulling bfdf757 on kurtmckee:skip-install-for-coverage-reporting
into ceb7141 on LudovicRousseau:master.

[LudovicRousseau]

make coverage takes 1.815s for me. 7 seconds is too slow.

I already reverted your changes in e6917e3
Why do you want to use tox here?

[kurtmckee]

Because tox installs the package and ensures that developers are testing what they think they're testing.

I'll take out the Makefile change, though I recommend against it.

[LudovicRousseau]

I install the package using this Makefile rule:

install: clean
	$(PYTHON) -m pip install --editable .

So I am always using the current source code.