Improve the error message in a Y061 edge case (#445)

Currently we're recommending code that we'd immediately complain about
if the user actually used it. We shouldn't do that. Possibly we could
introduce another error code emitting an error for duplicate `Literal`
members, but for now I'd just like to fix the Y061 error message.

pyi.py

@@ -1432,7 +1432,9 @@ def _visit_typing_Literal(self, node: ast.Subscript) -> None:
             elts = node.slice.elts
             for i, elt in enumerate(elts):
                 if _is_None(elt):
-                    elts_without_none = elts[:i] + elts[i + 1 :]
+                    elts_without_none = elts[:i] + [
+                        elt for elt in elts[i + 1 :] if not _is_None(elt)
+                    ]
                     if len(elts_without_none) == 1:
                         new_literal_slice = unparse(elts_without_none[0])
                     else:

tests/literals.pyi

@@ -2,4 +2,4 @@ from typing import Literal
 
 Literal[None]  # Y061 None inside "Literal[]" expression. Replace with "None"
 Literal[True, None]  # Y061 None inside "Literal[]" expression. Replace with "Literal[True] | None"
-Literal[1, None, "foo", None]  # Y061 None inside "Literal[]" expression. Replace with "Literal[1, 'foo', None] | None"
+Literal[1, None, "foo", None]  # Y061 None inside "Literal[]" expression. Replace with "Literal[1, 'foo'] | None"