Itemadapter

`ItemAdapter.field_names` now returns a `KeysView` instead of a `list`.

Minor CI and test changes.

`ItemAdapter.get_field_meta` now returns an empty `MappingProxyType` object for
items without metadata support, instead of raising `TypeError`.

Improved the README and some docstrings.

Provided full test coverage, and refactored CI configuration, test
configuration and tests themselves.

Added support for Scrapy’s `BaseItem`.

Refactored and extended tests.

Code style and documentation fixes.

Removed support for `MutableMapping`.

Removed metadata support for arbitrary mutable mappings.

Rebuild for the Python Package Index.