I reproduced manually the issue. test_makeport() can trigger the "Uncaught thread exception: Exception".

test_makeport (test.test_ftplib.TestTLS_FTPClassMixin) ...

Warning -- Uncaught thread exception: Exception
Exception in thread Thread-67:
Traceback (most recent call last):
  File "/home/vstinner/python/main/Lib/asyncore.py", line 90, in read
    obj.handle_read_event()
    ^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/vstinner/python/main/Lib/test/test_ftplib.py", line 384, in handle_read_event
    self._do_ssl_handshake()
    ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/vstinner/python/main/Lib/test/test_ftplib.py", line 345, in _do_ssl_handshake
    self.socket.do_handshake()
    ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/vstinner/python/main/Lib/ssl.py", line 1346, in do_handshake
    self._sslobj.do_handshake()
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^
ssl.SSLZeroReturnError: TLS/SSL connection has been closed (EOF) (_ssl.c:998)

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/vstinner/python/main/Lib/threading.py", line 1031, in _bootstrap_inner
    self.run()
    ^^^^^^^^^^
  File "/home/vstinner/python/main/Lib/test/test_ftplib.py", line 298, in run
    asyncore.loop(timeout=0.1, count=1)
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/vstinner/python/main/Lib/asyncore.py", line 214, in loop
    poll_fun(timeout, map)
    ^^^^^^^^^^^^^^^^^^^^^^
  File "/home/vstinner/python/main/Lib/asyncore.py", line 157, in poll
    read(obj)
    ^^^^^^^^^
  File "/home/vstinner/python/main/Lib/asyncore.py", line 94, in read
    obj.handle_error()
    ^^^^^^^^^^^^^^^^^^
  File "/home/vstinner/python/main/Lib/test/test_ftplib.py", line 421, in handle_error
    raise Exception
    ^^^^^^^^^^^^^^^
Exception

ok