Canonicaljson

* `frozendict` is now an optional dependency; it is no longer required.

* Switch CI from Travis to Github Actions
* Add code to handle frozendict implementations using c-extension
* Add tests for Python 3.10
* Remove outdated Debian packaging code

* Fix producing non-standard JSON for Infinity, -Infinity, and NaN. This could
cause errors when encoding objects into canonical JSON that previously used to
work, but were incompatible with JSON implementations in other languages.
* Use UTF-8 to fix ASCII encoding errors when data containing Unicode was
attempted to be pretty-printed.

* The minimum version of simplejson was bumped to 3.14.0.
* Obsolete workaround for slow encoding of Unicode characters was removed.
* New APIs were added to iteratively encode JSON.

* JSON from the standard library is used automatically on PyPy.
* Support for Python versions which are end-of-lifed was dropped, Python >= 3.5
is supported and tested in continuous integration.
* An API to configure the underlying JSON library was added (`set_json_library`).

* Fix error when encoding non-BMP characters on UCS-2 python builds
(fixes issue 12).