Support loading of `.pyc` files

Currently the process for loading the file run the file through compile and then exec.

https://github.com/wemake-services/django-split-settings/blob/d7ca866f22ff642e59976a099547047823f3259e/split_settings/tools.py#L111-L114

This works perfectly for .py files (thanks for the project, by the way, it is really awesome).

I have a project where the files are precompiled into .pyc during building the production version, and all .py files are deleted from the codebase.

This causes two issues:

1. The names in the include call are no longer present. They've been deleted in a previous build step.
2. Even if I use a workaround for issue 1, the .pyc files do not work with the compile + exec approach.

I would like to request the ability to load .pyc files by addressing both of these points.

1. Allowing scripts to have a fallback, that is only loaded if the file is not found. Maybe that can be built into the optional implementation.

   include(
       optional("abc.py", "abc.pyc"),
       # ...
   )
   

2. Allowing .pyc files to be loaded. I've done it locally using this.

   import marshal
   import types
   
   with open(included_file, 'rb') as to_compile:
       to_compile.seek(16)
       compiled_code = marshal.load(to_compile)
       if isinstance(compiled_code, types.CodeType):
           exec(compiled_code, scope)
   

Start in split_settings/tools.py at the linked loading code and then trace the existing optional implementation. Determine how the include API should express a fallback and how the loader should handle the .pyc format; done means both fallback includes and precompiled files load successfully without their source files.