fix: DefinedNamespace: fixed handling of _NS attribute

This patch changes `DefinedNamespace` to always raise `AttributeError` for
the name `_NS` from `__getattr__`.

Without doign this `inspect.signature` recurses
infinitely when inspecting `rdflib.namespace.DefinedNamespace`.

One situation in which this occurs is when sphinx autodoc is generating
documentation from type hints:

```
WARNING: error while formatting signature for rdflib.namespace.DefinedNamespace: Handler <function record_typehints at 0x7fbf2696dd40> for event 'autodoc-process-signature' threw an exception (exception: maximum recursion depth exceeded while calling a Python object)
```

Also:
- fix: handling of `_NS` in `__contains__`. Without this `dir` on an empty
  `DefinedNamespace` does not behave correctly.

rdflib/namespace/__init__.py

@@ -3,7 +3,7 @@
 import warnings
 from functools import lru_cache
 from pathlib import Path
-from typing import TYPE_CHECKING, Any, Dict, Iterable, List, Optional, Tuple, Union
+from typing import TYPE_CHECKING, Any, Dict, Iterable, List, Optional, Set, Tuple, Union
 from unicodedata import category
 from urllib.parse import urldefrag, urljoin
 
@@ -190,6 +190,13 @@ def __repr__(self) -> str:
         return f"URIPattern({super().__repr__()})"
 
 
+# _DFNS_RESERVED_ATTRS are attributes for which DefinedNamespaceMeta should
+# always raise AttributeError if they are not defined.
+_DFNS_RESERVED_ATTRS: Set[str] = {
+    "_NS",
+}
+
+
 class DefinedNamespaceMeta(type):
     """Utility metaclass for generating URIRefs with a common prefix."""
 
@@ -215,10 +222,14 @@ def __getitem__(cls, name: str, default=None) -> URIRef:
         return cls._NS[name]
 
     def __getattr__(cls, name: str):
+        if name in _DFNS_RESERVED_ATTRS:
+            raise AttributeError(
+                f"DefinedNamespace like object has no attribute {name!r}"
+            )
         return cls.__getitem__(name)
 
     def __repr__(cls) -> str:
-        return f'Namespace("{cls._NS}")'
+        return f'Namespace({str(cls._NS)!r})'
 
     def __str__(cls) -> str:
         return str(cls._NS)
@@ -229,7 +240,7 @@ def __add__(cls, other: str) -> URIRef:
     def __contains__(cls, item: str) -> bool:
         """Determine whether a URI or an individual item belongs to this namespace"""
         item_str = str(item)
-        if item_str.startswith("__"):
+        if item_str.startswith("__") or item_str in _DFNS_RESERVED_ATTRS:
             return super().__contains__(item)  # type: ignore[misc] # undefined in superclass
         if item_str.startswith(str(cls._NS)):
             item_str = item_str[len(str(cls._NS)) :]