Remove importlib_metadata as imported by pytest. Fixes #322.

Author: jaraco

conftest.py

@@ -1,4 +1,25 @@
+import sys
+
+
 collect_ignore = [
     # this module fails mypy tests because 'setup.py' matches './setup.py'
     'prepare/example/setup.py',
 ]
+
+
+def pytest_configure():
+    remove_importlib_metadata()
+
+
+def remove_importlib_metadata():
+    """
+    Because pytest imports importlib_metadata, the coverage
+    reports are broken (#322). So work around the issue by
+    undoing the changes made by pytest's import of
+    importlib_metadata (if any).
+    """
+    if sys.meta_path[-1].__class__.__name__ == 'MetadataPathFinder':
+        del sys.meta_path[-1]
+    for mod in list(sys.modules):
+        if mod.startswith('importlib_metadata'):
+            del sys.modules[mod]