bpo-41025: Fix subclassing for zoneinfo.ZoneInfo (GH-20965)

Prior to this change, attempting to subclass the C implementation of
zoneinfo.ZoneInfo gave the following error:

    TypeError: unbound method ZoneInfo.__init_subclass__() needs an argument

https://bugs.python.org/issue41025

Author: pganssle

Lib/test/test_zoneinfo/test_zoneinfo.py

@@ -463,7 +463,7 @@ class CZoneInfoDatetimeSubclassTest(DatetimeSubclassMixin, CZoneInfoTest):
     pass
 
 
-class ZoneInfoTestSubclass(ZoneInfoTest):
+class ZoneInfoSubclassTest(ZoneInfoTest):
     @classmethod
     def setUpClass(cls):
         super().setUpClass()
@@ -484,7 +484,7 @@ def test_subclass_own_cache(self):
         self.assertIsInstance(sub_obj, self.klass)
 
 
-class CZoneInfoTestSubclass(ZoneInfoTest):
+class CZoneInfoSubclassTest(ZoneInfoSubclassTest):
     module = c_zoneinfo
 
 

Misc/NEWS.d/next/Library/2020-06-18-10-34-59.bpo-41025.elf_nz.rst

@@ -0,0 +1,2 @@
+Fixed an issue preventing the C implementation of :class:`zoneinfo.ZoneInfo`
+from being subclassed.

Modules/_zoneinfo.c

@@ -2557,7 +2557,7 @@ static PyMethodDef zoneinfo_methods[] = {
     {"_unpickle", (PyCFunction)zoneinfo__unpickle, METH_VARARGS | METH_CLASS,
      PyDoc_STR("Private method used in unpickling.")},
     {"__init_subclass__", (PyCFunction)(void (*)(void))zoneinfo_init_subclass,
-     METH_VARARGS | METH_KEYWORDS,
+     METH_VARARGS | METH_KEYWORDS | METH_CLASS,
      PyDoc_STR("Function to initialize subclasses.")},
     {NULL} /* Sentinel */
 };