Pinn waitress = 2.1.2 in Python 3.7 and 3.8 to fix robot tests

waitress = 2.1.2 fix the error:

   OSError: [Errno 9] Bad file descriptor

See: Pylons/waitress#374

This error causes the server to "freeze" on robot tests and the tests
never finish.

test_plone52.cfg

@@ -7,3 +7,15 @@ extends =
 
 [versions:python27]
 lazy-object-proxy = 1.6.0
+
+# FIXME: waitress = 2.1.2 fix the error:
+#     OSError: [Errno 9] Bad file descriptor
+# See: https://github.com/Pylons/waitress/issues/374
+# This error causes the server to "freeze" on robot tests and the tests never finish.
+# waitress = 2.1.2 has been pinned in Zope 4.8.2. So this pinn here should be removed
+# when Plone 5.2.9 is released.
+[versions:python37]
+waitress = 2.1.2
+
+[versions:python38]
+waitress = 2.1.2