Allow string indexing into Record (#501)

* Allow string indexing into Record

The Record interface inherits from Sequence which only supports integer indexing. The Postgres backend supports string indexing into Records (https://github.com/encode/databases/blob/master/databases/backends/postgres.py#L135).  This PR updates the interface to reflect that.

At least on the Postgres backend __getitem__ deserializes some data types, so it's not equivalent to the _mapping method.

databases/interfaces.py

@@ -73,3 +73,6 @@ class Record(Sequence):
     @property
     def _mapping(self) -> typing.Mapping:
         raise NotImplementedError()  # pragma: no cover
+
+    def __getitem__(self, key: typing.Any) -> typing.Any:
+        raise NotImplementedError()  # pragma: no cover