fix pep8

Author: charlesdong1991

pandas/core/strings.py

@@ -551,7 +551,7 @@ def str_replace(arr, pat, repl, n=-1, case=None, flags=0, regex=None):
 
     is_compiled_re = is_re(pat)
     if not is_compiled_re and regex is None:
-        if len(pat) == 1 and re.findall("\W", pat):
+        if len(pat) == 1 and re.findall(r"\W", pat):
             regex = False
             warnings.warn(f"{pat} is interpreted as a literal in default, "
                           f"not regex. The default will change in the future",