Fix coverage when running tox -p auto

This is a workaround for tox-dev/tox#1536.

Author: mgedmin

.coveragerc

@@ -1,5 +1,11 @@
 [run]
-source = src
+source = zope.file
+
+[paths]
+source =
+    src/
+    .tox/*/lib/python*/site-packages/
+    .tox/pypy*/site-packages/
 
 [report]
 exclude_lines =

tox.ini

@@ -14,10 +14,10 @@ deps =
     zope.testing
 
 [testenv:coverage]
-usedevelop = true
 basepython = python3
 deps =
     coverage
 commands =
-    coverage run -m zope.testrunner --test-path=src []
-    coverage report --fail-under=96
+    coverage run -p -m zope.testrunner --test-path=src []
+    coverage combine
+    coverage report --fail-under=97