Fix entry point script.

[Hellseher]

main.py is in a "mps_youtube" and not in "mps_youtube/main".

[McSinyx]

To quote the entry point specification:

The object reference points to a Python object. It is either in the form importable.module, or importable.module:object.attr. Each of the parts delimited by dots and the colon is a valid Python identifier. It is intended to be looked up like this:

import importlib
modname, qualname_separator, qualname = object_ref.partition(':')
obj = importlib.import_module(modname)
if qualname_separator:
    for attr in qualname.split('.'):
        obj = getattr(obj, attr)

In other words, even though some build frontends like pip create entry points with from importable.module import object; object.attr()/from mps_youtube import main; main.main(), they only need to from importable import module; module.object.attr/import mps_youtube; mps_youtube.main.main to be compliant with the specification.