typing: add some assertions that a variable isn't None

In the case of blackbox, there's a default limit if one isn't explicitly
supplied.  For the monotone regex, neither group is optional, so a match means
it's not None.

hgext/blackbox.py

@@ -217,6 +217,8 def blackbox(ui, repo, *revs, **opts):
         return
 
     limit = opts.get('limit')
+    assert limit is not None  # help pytype
+
     fp = repo.vfs(b'blackbox.log', b'r')
     lines = fp.read().split(b'\n')
 

hgext/convert/monotone.py

@@ -243,6 +243,7 class monotone_source(common.converter_s
             m = self.cert_re.match(e)
             if m:
                 name, value = m.groups()
+                assert value is not None  # help pytype
                 value = value.replace(br'\"', b'"')
                 value = value.replace(br'\\', b'\\')
                 certs[name] = value