Allow use of orjson instead of json

The difference in speed seems negligible at start up, which is when
most time is taken for actually parsing JSON. I could potentially be
missing something but profiling didn't point to any discernable
difference.

setup.py

@@ -39,6 +39,9 @@ extras_require = {
         'sphinx==4.0.2',
         'sphinxcontrib_trio==1.1.2',
         'sphinxcontrib-websupport',
+    ],
+    'speed': [
+        'orjson>=3.5.4',
     ]
 }