fix `DeprecationWarning: invalid escape sequence '\s'` for whitespace filter (#2560)

* fix `DeprecationWarning: invalid escape sequence '\s'`

* add type hints

* Revert "add type hints"

This reverts commit 15d8abc626a84e97f8c238ddfbf9e243d6f6eb5c.

lm_eval/filters/extraction.py

@@ -164,7 +164,7 @@ class MultiChoiceRegexFilter(RegexFilter):
             fallback_regex = re.compile("|".join(fallback_regexes))
             without_paren_fallback_regex = "|".join(without_paren_fallback_regexes)
             without_paren_fallback_regex = re.compile(
-                f":[\s]*({without_paren_fallback_regex})"
+                rf":[\s]*({without_paren_fallback_regex})"
             )
 
             filtered = []