patch py-mccabe and py-flake8 to not require pytest-runner for install (#5427)

- Filter the pytest-runner dependency out of setup.py in py-mccabe and
  py-flake8

var/spack/repos/builtin/packages/py-flake8/package.py

@@ -66,3 +66,7 @@ class PyFlake8(PythonPackage):
 
     # TODO: Add test dependencies
     # depends_on('py-nose', type='test')
+
+    def patch(self):
+        """Filter pytest-runner requirement out of setup.py."""
+        filter_file("['pytest-runner']", "[]", 'setup.py', string=True)

var/spack/repos/builtin/packages/py-mccabe/package.py

@@ -47,3 +47,7 @@ class PyMccabe(PythonPackage):
 
     # TODO: Add test dependencies
     # depends_on('py-pytest', type='test')
+
+    def patch(self):
+        """Filter pytest-runner requirement out of setup.py."""
+        filter_file("['pytest-runner']", "[]", 'setup.py', string=True)