I've attached issue14455-v2.txt with an updated patch. The patch is still very much a work in progress, I haven't had as much time to work on this as I'd like.

This version:
* Should apply cleanly to the tip of the default branch

* Move around some code.

* Doesn't pass unit tests (most likely because I've botched the manual
  merge). The unittests also don't cover the functionality I've added.

* Adds documentation

* Adds 'skipkeys' and 'sort_keys' to the write functions (with the 
  same semantics as these keywords have with json.dump)

* Adds 'data_as_bytes' to the read functions. Then this option is true
  binary data is returned as an instance of bytes instead of plistlib.Data
  The latter is still the default.