fix tox -e *-coverage running against source files

Author: graingert

tox.ini

@@ -12,7 +12,7 @@ skip_missing_interpreters = true
 
 [testenv]
 setenv =
-    coverage: _PYTEST_CMD=coverage run --append -m pytest
+    coverage: _PYTEST_CMD=pytest --cov
     xdist: _PYTEST_MORE_ARGS=-n3 -rfsxX
 deps =
     pytestlatest: pytest
@@ -29,7 +29,7 @@ deps =
     pytest44: pytest~=4.4.0
     pytest43: pytest~=4.3.0
 
-    coverage: coverage
+    coverage: pytest-cov
     xdist: pytest-xdist
     -r{toxinidir}/requirements-testing.txt
     anyio[trio]