Add stub for html._replace_charref from standard library

`_replace_charref` is defined here: https://github.com/python/cpython/blob/64fab74bd7288bfa67cd7727452febdaafed4270/Lib/html/__init__.py#L91-L115

Although it’s a private attribute we have a use case for importing it: We re-implement `html.unescape` using our own definition of `_charref`, but we don’t want to reimpliment `_replace_charref`. Example can be found [in our codebase](https://github.com/alphagov/notifications-utils/blob/1eccc101a96cf120ce7bdc71528222ebc694432b/notifications_utils/formatters.py#L161-L167).

Author: quis

stdlib/html/__init__.pyi

@@ -1,4 +1,7 @@
+import re
+
 __all__ = ["escape", "unescape"]
 
 def escape(s: str, quote: bool = True) -> str: ...
 def unescape(s: str) -> str: ...
+def _replace_charref(s: re.Match[str]) -> str: ...