tox: Update pytest-cov to 2.0.0.

Changelog:

* Added --cov-fail-under, akin to the new fail_under option in coverage-4.0
  (automatically activated if there's a [report] fail_under = ... in
  .coveragerc).
* Changed --cov-report=term to automatically upgrade to
  --cov-report=term-missing if there's [run] show_missing = True in
  .coveragerc.
* Changed --cov so it can be used with no path argument (in wich case the
  source settings from .coveragerc will be used instead).
* Fixed .pth installation to work in all cases (install, easy_install, wheels,
  develop etc).
* Fixed .pth uninstallation to work for wheel installs.
* Support for coverage 4.0.
* Data file suffixing changed to use coverage's data_suffix=True option
  (instead of the custom suffixing).
* Avoid warning about missing coverage data (just like
  coverage.control.process_startup).
* Fixed a race condition when running with xdist (all the workers tried to
  combine the files). It's possible that this issue is not present in
  pytest-cov 1.8.X.

tox.ini

@@ -61,7 +61,7 @@ passenv = PYTHON DISPLAY XAUTHORITY HOME
 deps =
     {[testenv:unittests]deps}
     coverage==3.7.1
-    pytest-cov==1.8.1
+    pytest-cov==2.0.0
     cov-core==1.15.0
 commands =
     {envpython} scripts/link_pyqt.py --tox {envdir}