Don't crash on double-underscore-prefixed keyword arg (#6736)

This avoids the crash reported in #6727. However that issue should remain open, because there's a logic bug somewhere (but it's not high enough priority to try and track it down).

Author: Guido van Rossum

mypy/checkexpr.py

@@ -1138,8 +1138,7 @@ def check_argument_count(self,
                                               not is_unexpected_arg_error):
                 # No actual for a mandatory named formal
                 if messages:
-                    argname = callee.arg_names[i]
-                    assert argname is not None
+                    argname = callee.arg_names[i] or "?"
                     messages.missing_named_argument(callee, context, argname)
                 ok = False
             elif kind in [nodes.ARG_POS, nodes.ARG_OPT,