Mock more modules for RTD

Also add a requirements.txt for RTD so that it will install Mako into
the virtualenv when generating the documentation.

docs/conf.py

@@ -377,6 +377,6 @@ class Mock(object):
     def __getitem__(cls, key):
         return cls.__getattr__(key)
 
-MOCK_MODULES = ["selinux"]
+MOCK_MODULES = ["selinux", "dnf", "rpmUtils", "rpmUtils.arch"]
 for mod_name in MOCK_MODULES:
     sys.modules[mod_name] = Mock()

requirements.txt

@@ -0,0 +1 @@
+Mako