C API/GC: datetime.datetime doesn't have `Py_TPFLAGS_HAVE_GC`, potentially causing leaks in `tzinfo` cycles

Bug report

Bug description:

The GC docs say (emphasis mine)

Python’s support for detecting and collecting garbage which involves circular references requires support from object types which are “containers” for other objects which may also be containers. Types which do not store references to other objects, or which only store references to atomic types (such as numbers or strings), do not need to provide any explicit support for garbage collection.

However in datetime, PyDateTime_DateTimeType is a container (of tzinfo), but doesn’t have the GC flags set.

With help on Python's discuss forums, I was able to get to this minimal example that appears to show a possible leak.

from datetime import datetime, tzinfo
import gc, sys

class MyTzinfo(tzinfo): pass

tz = MyTzinfo()
dt = datetime(2000, 1, 1, tzinfo=tz)
tz.foo = dt  # circular ref introduced here
assert sys.getrefcount(tz) == 3 # 1 from dt, 1 variable, 1extra from passing it in
assert sys.getrefcount(tz) == 3 # 1 from tz, 1 variable, 1extra from passing it in
assert len([obj for obj in gc.get_objects() if isinstance(obj, MyTzinfo)]) == 1
del tz
assert len([obj for obj in gc.get_objects() if isinstance(obj, MyTzinfo)]) == 1
del dt
gc.collect()
# At this point, dt and tz should be gone
assert len([obj for obj in gc.get_objects() if isinstance(obj, MyTzinfo)]) == 0

If datetime is replaced with a dummy class, the above program works:

class datetime:
    def __init__(self, *args, **kwargs):
        self.args = args
        self.kwargs = kwargs

I'm happy to submit a PR if needed.

CPython versions tested on:

3.12

Operating systems tested on:

macOS

Start in Modules/_datetimemodule.c by examining PyDateTime_DateTimeType and how its tzinfo reference interacts with garbage-collection support. Run the minimal datetime/tzinfo cycle reproducer and inspect existing datetime tests. Done means the cycle is collected after gc.collect(), with a regression test covering the behavior.