coverage: lock down to coverage 4

pytest-cov has issues when run in parallel when using coverage 5,
as noted by the pytest-cov authors.

Use coverage 4 until pytest-cov is ready for coverage 5, or a
better solution is documented.

pytest-dev/pytest-cov@d3daf76

pytest-dev/pytest-cov#386

setup.cfg

@@ -23,6 +23,7 @@ setup_requires = setuptools_scm[toml] >= 3.4.1
 [options.extras_require]
 testing =
 	# upstream
+	coverage < 5
 	pytest >= 3.5, !=3.7.3
 	pytest-checkdocs >= 1.2.3
 	pytest-flake8

tox.ini

@@ -20,7 +20,7 @@ usedevelop = True
 extras = testing
 
 [testenv:clean]
-deps = coverage
+deps = coverage<5
 depends =
 skip_install = true
 commands = coverage erase