test requirements: Update pytest-cov to 2.3.1

- Fixed regression causing spurious errors when xdist was used.
- Fixed DeprecationWarning about incorrect addoption use.
- Fixed deprecated use of funcarg fixture API.

misc/requirements/requirements-tests.txt

@@ -18,7 +18,7 @@ py==1.4.31
 pytest==2.9.2
 pytest-bdd==2.17.0
 pytest-catchlog==1.2.2
-pytest-cov==2.3.0
+pytest-cov==2.3.1
 pytest-faulthandler==1.3.0
 pytest-instafail==0.3.0
 pytest-mock==1.2