Revert tomllib change

Author: vstinner

Lib/tomllib/_parser.py

@@ -4,6 +4,8 @@
 
 from __future__ import annotations
 
+from types import MappingProxyType
+
 from ._re import (
     RE_DATETIME,
     RE_LOCALTIME,
@@ -40,7 +42,7 @@
 KEY_INITIAL_CHARS: Final = BARE_KEY_CHARS | frozenset("\"'")
 HEXDIGIT_CHARS: Final = frozenset("abcdef" "ABCDEF" "0123456789")
 
-BASIC_STR_ESCAPE_REPLACEMENTS: Final = frozendict(
+BASIC_STR_ESCAPE_REPLACEMENTS: Final = MappingProxyType(
     {
         "\\b": "\u0008",  # backspace
         "\\t": "\u0009",  # tab