fix: DefinedNamespace: always raise AttributeError for _partialmethod

This patch changes `DefinedNamespace` to always raise `AttributeError` if
`_partialmethod` is not defined, even if
`DefinedNamespace._fail == False`. This because this property is
accessed by `inspect.signature` when building docs and if `AttributeError` is not
raised when this property is accessed it results in infinite recursion.

```
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)
```

rdflib/namespace/__init__.py

@@ -190,6 +190,18 @@ def __repr__(self) -> str:
         return f"URIPattern({super().__repr__()})"
 
 
+# _RESERVED_ATTRS are attributes for which AttributeError should always be
+# raised if they are not defined.
+_RESERVED_ATTRS = {
+    # `_partialmethod` is being accessed by `inspect.signature` (see
+    # https://github.com/python/cpython/blob/72965981d1128b3923dad5e850c8cff626ae4dc7/Lib/inspect.py#L2428-L2431)
+    #  and if it does not return AttributeError inspect behaves
+    #  incorrectly. This results in warnings in sphinx which are being
+    #  treated as errors.
+    "_partialmethod",
+}
+
+
 class DefinedNamespaceMeta(type):
     """Utility metaclass for generating URIRefs with a common prefix."""
 
@@ -204,8 +216,9 @@ def __getitem__(cls, name: str, default=None) -> URIRef:
         name = str(name)
         if str(name).startswith("__"):
             return super().__getitem__(name, default)  # type: ignore[misc] # undefined in superclass
-        if (cls._warn or cls._fail) and name not in cls:
-            if cls._fail:
+
+        if (cls._warn or cls._fail or name in _RESERVED_ATTRS) and name not in cls:
+            if cls._fail or name in _RESERVED_ATTRS:
                 raise AttributeError(f"term '{name}' not in namespace '{cls._NS}'")
             else:
                 warnings.warn(

test/test_namespace/test_definednamespace.py

@@ -0,0 +1,27 @@
+import pytest
+
+from rdflib import BRICK
+
+NOFAIL_DEFINED_NAMESPACE = BRICK
+
+
+def test_nofail_attr() -> None:
+    """
+    Undefined attributes do not raise an exception if `DefinedNamespace._fail`
+    is `False`.
+    """
+    assert not NOFAIL_DEFINED_NAMESPACE._fail
+    assert (
+        f"{NOFAIL_DEFINED_NAMESPACE}not_defined_in_namespace"
+        == f"{NOFAIL_DEFINED_NAMESPACE.not_defined_in_namespace}"
+    )
+
+
+def test_nofail_partialmethod_attrerror() -> None:
+    """
+    Accessing `_partialmethod` raises an exception even if
+    `DefinedNamespace._fail` is `False`.
+    """
+    assert not NOFAIL_DEFINED_NAMESPACE._fail
+    with pytest.raises(AttributeError):
+        _ = NOFAIL_DEFINED_NAMESPACE._partialmethod