fix: revert strenum change

Signed-off-by: Henry Schreiner <[email protected]>

Author: henryiii

src/boost_histogram/histogram.py

@@ -55,10 +55,13 @@
     raise new_exception from err
 
 
-class Kind(enum.StrEnum):
+# This is a StrEnum as defined in Python 3.11
+class Kind(str, enum.Enum):
     COUNT = "COUNT"
     MEAN = "MEAN"
 
+    __str__ = str.__str__
+
 
 __all__ = [
     "Histogram",