Cover missing } at the end of f-strings

Author: pablogsal

Grammar/python.gram

@@ -1363,6 +1363,9 @@ invalid_replacement_field:
     | '{' a='!' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "f-string: expression required before '!'") }
     | '{' a='}' { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "f-string: empty expression not allowed") }
     | '{' (yield_expr | star_expressions) "="? invalid_conversion_character
+    | '{' (yield_expr | star_expressions) "="? [ "!" NAME ] [ ':' fstring_format_spec* ] !'}' {
+         RAISE_SYNTAX_ERROR("f-string: expecting '}'")
+    }
 invalid_conversion_character:
     | a="!" &(':'|'}') { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "f-string: missed conversion character") }
     | a="!" !NAME { RAISE_SYNTAX_ERROR_KNOWN_LOCATION(a, "f-string: invalid conversion character") }