bpo-35925: Skip SSL tests that fail due to weak external certs. (GH-13124)

Modern Linux distros such as Debian Buster have default OpenSSL system
configurations that reject connections to servers with weak certificates
by default.  This causes our test suite run with external networking
resources enabled to skip these tests when they encounter such a failure.

Fixing the network servers is a separate issue.

Lib/test/test_httplib.py

@@ -4,6 +4,7 @@ import io
 import itertools
 import os
 import array
+import re
 import socket
 import threading
 
@@ -1619,14 +1620,30 @@ class HTTPSTest(TestCase):
         # We feed the server's cert as a validating cert
         import ssl
         support.requires('network')
-        with support.transient_internet('self-signed.pythontest.net'):
+        selfsigned_pythontestdotnet = 'self-signed.pythontest.net'
+        with support.transient_internet(selfsigned_pythontestdotnet):
             context = ssl.SSLContext(ssl.PROTOCOL_TLS_CLIENT)
             self.assertEqual(context.verify_mode, ssl.CERT_REQUIRED)
             self.assertEqual(context.check_hostname, True)
             context.load_verify_locations(CERT_selfsigned_pythontestdotnet)
-            h = client.HTTPSConnection('self-signed.pythontest.net', 443, context=context)
-            h.request('GET', '/')
-            resp = h.getresponse()
+            try:
+                h = client.HTTPSConnection(selfsigned_pythontestdotnet, 443,
+                                           context=context)
+                h.request('GET', '/')
+                resp = h.getresponse()
+            except ssl.SSLError as ssl_err:
+                ssl_err_str = str(ssl_err)
+                # In the error message of [SSL: CERTIFICATE_VERIFY_FAILED] on
+                # modern Linux distros (Debian Buster, etc) default OpenSSL
+                # configurations it'll fail saying "key too weak" until we
+                # address https://bugs.python.org/issue36816 to use a proper
+                # key size on self-signed.pythontest.net.
+                if re.search(r'(?i)key.too.weak', ssl_err_str):
+                    raise unittest.SkipTest(
+                        f'Got {ssl_err_str} trying to connect '
+                        f'to {selfsigned_pythontestdotnet}. '
+                        'See https://bugs.python.org/issue36816.')
+                raise
             server_string = resp.getheader('server')
             resp.close()
             h.close()