Canonicaljson

Additions:

* Add a generic `register_preserialisation_callback` mechanism, which
allows users to teach canonicaljson how to JSON-encode custom types.

Breaking changes:

* Remove support for serialising `frozendict` instances. Use the new
`register_preserialisation_callback` mechanism to replace this
functionality if needed.
* Remove support for `simplejson` and the `set_json_library`alternative
json libraries.

* Update type hints to pass under mypy 1.0.

* Remove unused `setuptools_scm` build requirement.

* Properly package the `canonicaljson` module.

* Bump version to 1.6.2

The v1.6.1 tag was created without having bumped the version in
`canonicaljson.py`. This means that installing from source at the `v1.6.1` tag will
install the package with the wrong version in its metadata. The 1.6.1 artefacts
uploaded to PyPI did have the correct version; `pip install`ing
directly from the archive resulted in a consistent version number.

* Add type annotations.