[3.14] gh-127146: Allow ignored keys to be missing in test_sysconfig (GH-135622) (#135650)

Fixes the test on Emscripten where userbase can be missing.
(cherry picked from commit 28c71ee)

Co-authored-by: Hood Chatham <roberthoodchatham@gmail.com>

Author: miss-islington

Lib/test/test_sysconfig.py

@@ -712,8 +712,8 @@ def test_sysconfigdata_json(self):
             ignore_keys |= {'prefix', 'exec_prefix', 'base', 'platbase', 'installed_base', 'installed_platbase'}
 
         for key in ignore_keys:
-            json_config_vars.pop(key)
-            system_config_vars.pop(key)
+            json_config_vars.pop(key, None)
+            system_config_vars.pop(key, None)
 
         self.assertEqual(system_config_vars, json_config_vars)