add support for pip install

[branchvincent]

Fixes #10

This add a pyproject.toml file (the successor to setup.py, favoring static metadata and allowing different "backends" beyond setuptools) so users can pip install it:

$ python3 -m venv .venv
$ .venv/bin/pip install git+https://github.com/branchvincent/littlecheck@pyproject
$ .venv/bin/littlecheck --help
usage: littlecheck [-h] [-s SUBSTITUTE] [-p] [--force-color] file [file ...]

littlecheck: command line tool tester.
...

As for the build-backend, I've chosen flit_core from the packaging guide because it is very simple and doesn't require custom configuration (unlike setuptools)

[zanchey]

Thanks, that's a good step. Possibly we should look at PyPA submission?

[branchvincent]

@zanchey sorry for the long delay, the name is available: https://pypi.org/search/?q=littlecheck

if you wanted to publish, it should be as simple as (assuming you have a PyPI account):

pipx run build
pipx run twine upload dist/*

Happy to help add a publishing step to CI if that would be useful