Fix installing entry points

[mgorny]

Not sure why or when but the string syntax for entry points does not
seem to work correctly anymore (no scripts are installed). Use
the explicit list-in-dict syntax instead.

[bbc2]

This seems to make sense. Do you have an example of an environment where the current setup.py doesn't work? Everything seems fine with Python 3.10.2 and setuptools 60.9.3.

If you have a way of reproducing the problem that would be great. Otherwise I'll probably just do some more testing and merge this since it looks like passing a dict with entry_points is the only example provided by setuptool's documentation (https://setuptools.pypa.io/en/latest/).