Catch dependency conflicts in testing (and update docs)

[tjd2002]

Per discussion at #921.

It is currently possible for our pinned requirements list to specify a set of packages that are in conflict with each other, and for tests to succeed. (This is because pip install does not fail in this case, see: pypa/pip#775 pypa/pip#988 )

Indeed, right now we have such a conflict (until #923 is merged). So adding this test should cause CI to (correctly) fail.

The pip check command checks for this condition, and fails if there are conflicts. This PR adds pip check to the tox testenv, and also recommends running it when initially writing the requirements files.

I am not sure if I put the command in the right place within tox, but I did test that it detects the conflict.

[codecov]

Codecov Report

Merging #925 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #925   +/-   ##
=======================================
  Coverage   73.01%   73.01%           
=======================================
  Files          36       36           
  Lines        2594     2594           
  Branches      494      494           
=======================================
  Hits         1894     1894           
  Misses        587      587           
  Partials      113      113

---

Continue to review full report at Codecov.

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[codecov]

Codecov Report

Merging #925 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev     #925   +/-   ##
=======================================
  Coverage   73.01%   73.01%           
=======================================
  Files          36       36           
  Lines        2594     2594           
  Branches      494      494           
=======================================
  Hits         1894     1894           
  Misses        587      587           
  Partials      113      113

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 355e8e0...a0e58c8. Read the comment docs.

[tjd2002]

Now that #923 is merged, CircleCI is passing as expected 🎉, but Appveyor fails for odd reasons:

Appveyor:py27 fails because virtualenv installs an old version of pip version (8.1.1) that doesn't support pip check. Updating pip within appveyor.yml doesn't help as pip seems to be provided by virtualenv.

Appveyor:py36 fails for a strange reason: it is still installing the conflicting urllib3 version (1.25.2) even though 1.24.3 is listed in requirements.txt and requirements-dev.txt.

Appveyor:py37 uses pip 10.0.1, and passes, so this is not a generic Windows problem.

I am going to try updating virtualenv itself within appveyor.yml to see if perhaps both of these problems will go away.

[tjd2002]

Hurray! Updating virtualenv within appveyor.yml gives us a more recent pip within the virtualenvs, and also fixes the strange dependency behavior I saw in Appveyor:py36.