Ignore ImportWarnings regarding package resolution

The problem is described/discussed in pytest-dev#3061

Ideally this should be a temporary solution until we find a proper one
which gets rid of the warning

tox.ini

@@ -215,6 +215,7 @@ filterwarnings =
     ignore:.*type argument to addoption.*:DeprecationWarning
     # produced by python >=3.5 on execnet (pytest-xdist)
     ignore:.*inspect.getargspec.*deprecated, use inspect.signature.*:DeprecationWarning
+    ignore:.*can't resolve package from __spec__ or __package__.*:ImportWarning
 
 [flake8]
 max-line-length = 120