Port setup.py to pyproject.toml

[benkonrath]

I get this warning when installing my personal integration branch of easy-thumbnails:

DEPRECATION: Building 'easy-thumbnails' using the legacy setup.py bdist_wheel mechanism, which
will be removed in a future version. pip 25.3 will enforce this behaviour change. A possible
replacement is to use the standardized build interface by setting the `--use-pep517` option,
(possibly combined with `--no-build-isolation`), or adding a `pyproject.toml` file to the
source tree of 'easy-thumbnails'. Discussion can be found at 
https://github.com/pypa/pip/issues/6334

Switching to pyproject.toml (this PR) fixes the issue for me.

[benkonrath]

I needed to manually include the directories inside the easy_thumbnails dir. Using setuptools-scm instead could also be an option, but the manual option seemed simpler.
https://pypi.org/project/setuptools-scm/