Add coverage to tox

Author: hugovk

tox.ini

@@ -15,6 +15,11 @@ extras =
 commands =
     {envpython} selftest.py
     {envpython} -m pytest \
+      --cov PIL \
+      --cov Tests \
+      --cov-report html \
+      --cov-report term \
+      --cov-report xml \
       --dist worksteal \
       --numprocesses auto \
       -W always \