start coverage before pytest

Author: ConorMacBride

tox.ini

@@ -58,7 +58,8 @@ commands =
     # Make sure the tests pass with and without --mpl
     # Use -m so pytest skips "subtests" which always apply --mpl
     pytest '{toxinidir}' -m "mpl_image_compare" {posargs}
-    pytest '{toxinidir}' --mpl --cov pytest_mpl {posargs}
+    coverage run --source=pytest_mpl -m pytest '{toxinidir}' --mpl
+    coverage xml -o '{toxinidir}{/}coverage.xml'
 
 [testenv:codestyle]
 skip_install = true