I can consistently product a ResourceWarning when running test_ssl:

./python -m test test_ssl
Run tests sequentially
0:00:00 [1/1] test_ssl
Exception in thread Thread-81:
Traceback (most recent call last):
  File "/home/angwer/cpython/Lib/threading.py", line 916, in _bootstrap_inner
    self.run()
  File "/home/angwer/cpython/Lib/test/test_ssl.py", line 1924, in run
    if not self.wrap_conn():
  File "/home/angwer/cpython/Lib/test/test_ssl.py", line 1869, in wrap_conn
    self.sock, server_side=True)
  File "/home/angwer/cpython/Lib/ssl.py", line 401, in wrap_socket
    _context=self, _session=session)
  File "/home/angwer/cpython/Lib/ssl.py", line 764, in __init__
    raise ValueError("check_hostname requires server_hostname")
ValueError: check_hostname requires server_hostname

/home/angwer/cpython/Lib/threading.py:916: ResourceWarning: unclosed <socket.socket fd=5, family=AddressFamily.AF_INET, type=SocketKind.SOCK_STREAM, proto=0, laddr=('127.0.0.1', 34271), raddr=('127.0.0.1', 34156)>
  self.run()
1 test OK.

Total duration: 1 sec
Tests result: SUCCESS

The problem lies in test_echo, using PROTOCOL_TLS_CLIENT as the server but doesn't not provide a hostname? I make a try which seems solving the problem but not sure this is what you want.

New changeset cf2689e191f8 by Christian Heimes in branch 'default':
Issue #28093: Check more invalid combinations of PROTOCOL_TLS_CLIENT / PROTOCOL_TLS_SERVER
https://hg.python.org/cpython/rev/cf2689e191f8

The warning is gone. Close this thread. :)