Itemadapter

Improve performance by removing imports inside functions
([60](https://github.com/scrapy/itemadapter/pull/60))

Added `ItemAdapter.is_item_class` and `ItemAdapter.get_field_meta_from_class`
([54](https://github.com/scrapy/itemadapter/pull/54))

Added built-in support for `pydantic` models ([53](https://github.com/scrapy/itemadapter/pull/53))

Adapter interface: added the ability to support arbitrary types,
by implementing a MutableMapping-based interface.
By way of this change, now any type can be used as a Scrapy item.

Dropped support for Python 3.5 (38).

The new `get_field_meta_from_class` function offers the same functionality as
`ItemAdapter.get_field_meta` but for an item class, as opposed to an item
object (34, 35).

`ItemAdapter.__repr__` no longer raises exceptions caused by the underlying
item (31, 41).

Minor improvement to the release process (37), and cleanup of test warnings (40).

Added `ItemAdapter.asdict`, which allows converting an item and all of its
nested items into `dict` objects (27, 29).

Improved `ItemAdapter` performance by reducing time complexity for lookups and
traversals for dataclass and attrs items (28).