It's looping in Lib/asyncore.py:poll

select(4, [3], [3], [3], {30, 0})       = 1 (out [3], left {29, 999994})
select(4, [3], [3], [3], {30, 0})       = 1 (out [3], left {29, 999994})
select(4, [3], [3], [3], {30, 0})       = 1 (out [3], left {29, 999994})

loop sets the Unix domain socket in the writable set, and contrarily to AF_INET/AF_INET6 sockets, bound AF_UNIX SOCK_STREAM sockets are reported as writable before any client connects to them, which triggers the loop.

I've attached a patch which just doesn't add the socket to the writable set if it's in the accepting state.
It fixes the loop, and doesn't seem to cause any regression in test_asyncore, but since it's the first time I'm looking at asyncore's code, I might very well have missed something :-)