refactor: eliminate unused imports

`XMLNS` was seen as unused in `rdflib/__init__.py` but this change adds it to
`__all__` in that module so that it is not unused.

This change also removes actual unused imports in
`rdflib/namespace/__init__.py`.

Author: aucampia

rdflib/__init__.py

@@ -92,6 +92,7 @@
     "TIME",
     "VANN",
     "VOID",
+    "XMLNS",
     "XSD",
     "util",
     "plugin",

rdflib/namespace/__init__.py

@@ -1,6 +1,4 @@
-import json
 import logging
-import sys
 import warnings
 from functools import lru_cache
 from pathlib import Path