Just built v3.7.0b1, and have the following test hangs (see attached).

My build is on Ubuntu 16.04;  lsb_release -a output:

LSB Version:	core-9.20160110ubuntu0.2-amd64:core-9.20160110ubuntu0.2-noarch:printing-9.20160110ubuntu0.2-amd64:printing-9.20160110ubuntu0.2-noarch:security-9.20160110ubuntu0.2-amd64:security-9.20160110ubuntu0.2-noarch
Distributor ID:	Ubuntu
Description:	Ubuntu 16.04.3 LTS
Release:	16.04
Codename:	xenial

My "usual" config / build process:

$ make distclean
$ ./configure --enable-shared --enable-loadable-sqlite-extensions --with-system-ffi --with-ensurepip=upgrade --enable-optimizations
$ make -j

I have been building 3.7 weekly, from master, and I've never seen anything like this before.

...also, my initial build (after inspecting git log) was from (as I usually do) master/HEAD;

To check that this was indeed from v3.7.0b1 release, I `distclean` and checked out the v3.7.0b1 tag, and rebuilt.   (Looks like same error / behavior).

It's a duplicate of #32706. The new cert validation code causes the handshake to terminate properly. The old asyncore based test routine is not able to handle this correctly when the machine is under heavy load.

On Ubuntu 17.10 (OpenSSL 1.0.2g), this patch fixes test_poplib error.
But I don't know this patch is right.

$ git diff
diff --git a/Lib/test/test_poplib.py b/Lib/test/test_poplib.py
index 6abdc93879..d7a8857b05 100644
--- a/Lib/test/test_poplib.py
+++ b/Lib/test/test_poplib.py
@@ -180,7 +180,8 @@ class DummyPOP3Handler(asynchat.async_chat):
                 elif err.args[0] == ssl.SSL_ERROR_EOF:
                     return self.handle_close()
                 # TODO: SSLError does not expose alert information
-                elif "SSLV3_ALERT_BAD_CERTIFICATE" in err.args[1]:
+                elif ("SSLV3_ALERT_BAD_CERTIFICATE" in err.args[1]
+                        or "SSLV3_ALERT_CERTIFICATE_UNKNOWN" in err.args[1]):
                     return self.handle_close()
                 raise
             except OSError as err: