PYTHON-4779 Use slots for Selection and TopologyDescription

[blink1073]

I ran a perf build: https://spruce.mongodb.com/task/mongo_python_driver_performance_benchmarks_perf_8.0_standalone_patch_578c6c2ad2559c4c939c682151eeeaf3b847ab3e_68643055c8c88400072d3e1b_25_07_01_19_00_56/trend-charts?execution=0

I did not see an appreciable change in FindOneByID8Threads or SmallDocInsertOne, unfortunately.

[ShaneHarvey]

Could you add something like this to the changelog?

- Classes :class:`~bson.int64.Int64`, :class:`~bson.min_key.MinKey`,
  :class:`~bson.max_key.MaxKey`, :class:`~bson.timestamp.Timestamp`,
  :class:`~bson.regex.Regex`, and :class:`~bson.dbref.DBRef` all implement
  ``__slots__`` now. This means that their attributes are fixed, and new
  attributes cannot be added to them at runtime.

Also is there any measurable benefit at all to this change? Like what if we have 1 thread just doing client._select_server? Or 100 threads?

[blink1073]

I see no significant differences with this script between this PR and master:

from pymongo import MongoClient
from pymongo.read_preferences import ReadPreference
from concurrent.futures import ThreadPoolExecutor, wait

client = MongoClient()
session = client.start_session()
read_preference = ReadPreference.PRIMARY


def test():
    for i in range(100):
        client._select_server(read_preference, session, "testOperation")



def test2_target():
    client._select_server(read_preference, session, "testOperation")


def test2():
    futures = []
    with ThreadPoolExecutor(max_workers=100) as pool:
        for i in range(100):
            futures.append(pool.submit(test2_target))
        wait(futures)
    

if __name__ == '__main__':
    import timeit
    print(timeit.timeit("test()", setup="from __main__ import test, client, read_preference, session", number=1000))
    print(timeit.timeit("test2()", setup="from __main__ import test2, client, read_preference, session", number=1000))

[blink1073]

Given that there is no measurable improvement, and Selection isn't even represented in our API docs, I think we should close this PR and ticket.

[ShaneHarvey]

Closing sounds good to me. We should only do this if we can show it leads to a measurable improvement.