Remove mutable default arg in function example (#944)

* Remove mutable default arg in function example

Removed mutable default arg, replaced with `None`.
https://docs.quantifiedcode.com/python-anti-patterns/correctness/mutable_default_value_as_argument.html

Replaced `dict()` with `{}`.
https://stackoverflow.com/questions/8424942/creating-a-new-dictionary-in-python

* Wrong import

* Undo last commit

My bad.

Co-authored-by: Hynek Schlawack <hs@ox.cx>

docs/extending.rst

@@ -176,10 +176,10 @@ Here are some tips for effective use of metadata:
     >>>
     >>> def typed(
     ...     cls, default=NOTHING, validator=None, repr=True,
-    ...     eq=True, order=None, hash=None, init=True, metadata={},
+    ...     eq=True, order=None, hash=None, init=True, metadata=None,
     ...     converter=None
     ... ):
-    ...     metadata = dict() if not metadata else metadata
+    ...     metadata = metadata or {}
     ...     metadata[MY_TYPE_METADATA] = cls
     ...     return field(
     ...         default=default, validator=validator, repr=repr,