> or left an outdated test somewhere that explicitly sets `reuse_address=False`

Looks like this was the issue, I left a `reuse_address=False` in both test_create_datagram_endpoint_nosoreuseport and test_create_datagram_endpoint_ip_addr. I fixed it locally on the latest commit to master:

[aeros:~/repos/cpython]$ ./python -Wall -m test test_asyncio -m test_create_datagram_endpoint_nosoreuseport -m test_create_datagram_endpoint_ip_addr 
0:00:00 load avg: 0.29 Run tests sequentially
0:00:00 load avg: 0.29 [1/1] test_asyncio

== Tests result: SUCCESS ==

1 test OK.

Total duration: 130 ms
Tests result: SUCCESS

I also used git grep to ensure `reuse_address=False` didn't exist at any other locations, other than the intentional one to test the DeprecationWarning.

But while doing that, I did notice that `reuse_address=` is present in several other locations through Lib/asyncio, which will likely add some confusing deprecation warnings to anyone running their asyncio programs with -Wall. The way we did the deprecation was using an _unset sentinel, so even `reuse_address=None` will cause a DeprecationWarning (since the parameter will be removed entirely in 3.11).

I'll fix them accordingly and open a new PR (as well as the backports).