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: