Add fragment property to URIRef (#1991)

Added a property `fragment` to `URIRef` so it is easy to access the URI's fragment:

```python
>>> URIRef("http://example.com/some/path/#some-fragment").fragment
'some-fragment'
>>> URIRef("http://example.com/some/path/").fragment
''
```

## Why is this useful?

The non-fragment part of the URI often just serves a namespace purpose, e.g. `https://brickschema.org/schema/Brick#`.

Actual "things" use that namespace, and have the "thing" name as the fragment, e.g. `https://brickschema.org/schema/Brick#Valve_Position_Sensor`

For display purposes, it's nice to get to the "thing" name, i.e. the fragment easily.

rdflib/term.py

@@ -314,6 +314,18 @@ def defrag(self) -> "URIRef":
         else:
             return self
 
+    @property
+    def fragment(self) -> str:
+        """
+        Return the URL Fragment
+
+        >>> URIRef("http://example.com/some/path/#some-fragment").fragment
+        'some-fragment'
+        >>> URIRef("http://example.com/some/path/").fragment
+        ''
+        """
+        return urlparse(self).fragment
+
     def __reduce__(self) -> Tuple[Type["URIRef"], Tuple[str]]:
         return (URIRef, (str(self),))