Add setuptools to build requirements

[jcushman]

This hopefully fixes the ModuleNotFoundError: No module named 'setuptools' error described in #14 when running pip install hyperscan.

I found this suggested fix on the poetry repo here. I don't really understand how pip/poetry/setuptools are interacting here or whether this is the right fix, but it seems to work.

[jcushman]

There's some more documentation of the [build-system] block here: https://pip.pypa.io/en/stable/reference/pip/#pep-517-and-518-support