Pylibjpeg

* Add encoding interface for plugins and update plugin requirements specification

* Better defined install requirements

* Added support for Python 3.9
* Removed standalone pydicom pixel data handler
* ``The pylibjpeg.pydicom`` module is deprecated and will be removed in v2.0,
use pydicom instead.
* ``get_pixel_data_decoders`` moved from ``pylibjpeg.pydicom.utils`` to
``pylibjpeg.utils``

* Only add a handler to pydicom on package import if no handler is already available

* Simplified the plugin interface
* Removed the `plugins` and `_config` modules
* Added `utils.get_decoders()` function
* Added `pydicom.utils.get_pixel_data_decoders()` function
* Changed arguments passed to pixel data decoding functions to `func(bytes, Dataset)`
* Changes to better support multiple plugins

* Fixed incorrect returned values for signed data when the precision is less than a round number of bytes