Resolve warning filter issues

There's an issue with 'pkgutils' and namespace packages. This has been
reported against setuptools [1], but until this is resolved, we simply
need to live with it. Ensure said warnings are filtered from tox too and
remove some unnecessary ones.

[1] https://github.com/pypa/setuptools/issues/1111

Signed-off-by: Stephen Finucane <stephen@that.guru>

tests/conftest.py

@@ -40,9 +40,7 @@ def _filter_warnings():
 
     ignore(category=DeprecationWarning, module='site')  # virtualenv
     ignore(category=PendingDeprecationWarning, module=r'_pytest\..*')
-    ignore(category=ImportWarning, module='backports')
     ignore(category=ImportWarning, module='pkgutil')
-    ignore(category=ImportWarning, module='pytest_cov')
 
 
 def _initialize_test_directory(session):

tox.ini

@@ -20,6 +20,7 @@ deps =
     du13: docutils==0.13.1
     du14: docutils==0.14
 setenv =
+    PYTHONWARNINGS = all,ignore::ImportWarning:pkgutil
     SPHINX_TEST_TEMPDIR = {envdir}/testbuild
 commands=
     pytest -Wall --durations 25 {posargs}