Drop the pickle/deepcopy singleton support of DEFAULT_FILL_VALUE

Nothing in the package pickles, and the sentinel is always resolved inside an
aggregate call.  The only case __reduce__ protected - transporting the
unresolved sentinel through pickle or deepcopy and passing it back in - is not
a realistic scenario; multiprocessing workers import the module themselves, so
the singleton exists natively in every process anyway.

Author: ml31415

numpy_groupies/aggregate_common.py

@@ -127,14 +127,6 @@ class _DefaultFillValue:
     def __repr__(self):
         return "DEFAULT_FILL_VALUE"
 
-    def __reduce__(self):
-        # keep the singleton through pickling and deepcopy
-        return (_get_default_fill_value, ())
-
-
-def _get_default_fill_value() -> _DefaultFillValue:
-    return DEFAULT_FILL_VALUE
-
 
 DEFAULT_FILL_VALUE = _DefaultFillValue()