gh-91928: Add datetime.UTC alias for datetime.timezone.utc (GH-91973)

### fixes #91928

`UTC` is now module attribute aliased to `datetime.timezone.utc`.
You can now do the following:
```python
from datetime import UTC
```

Doc/library/datetime.rst

@@ -84,6 +84,12 @@ The :mod:`datetime` module exports the following constants:
    The largest year number allowed in a :class:`date` or :class:`.datetime` object.
    :const:`MAXYEAR` is ``9999``.
 
+.. attribute:: UTC
+
+   Alias for the UTC timezone singleton :attr:`datetime.timezone.utc`.
+
+   .. versionadded:: 3.11
+
 Available Types
 ---------------