Allow disabling of python plugin

Fixes https://github.com/pytest-dev/pytest/issues/5277.

src/_pytest/assertion/rewrite.py

@@ -60,7 +60,10 @@ class AssertionRewritingHook(object):
 
     def __init__(self, config):
         self.config = config
-        self.fnpats = config.getini("python_files")
+        try:
+            self.fnpats = config.getini("python_files")
+        except ValueError:
+            self.fnpats = ["test_*.py", "*_test.py"]
         self.session = None
         self.modules = {}
         self._rewritten_names = set()