Update generic.py to repair the SyntaxWarning

when start bottles from CLI
I get `generic.py:95: SyntaxWarning: invalid escape sequence '\d'`

reference the [python doc](https://docs.python.org/3/reference/lexical_analysis.html#string-and-bytes-literals)

bottles/backend/utils/generic.py

@@ -94,7 +94,7 @@ def sort_by_version(_list: list, extra_check: str = "async"):
     def natural_keys(text):
         result = [int(re.search(extra_check, text) is None)]
         result.extend(
-            [int(t) if t.isdigit() else t.lower() for t in re.split("(\d+)", text)]
+            [int(t) if t.isdigit() else t.lower() for t in re.split(r"(\d+)", text)]
         )
         return result