Add pyproject.toml.

It's autogenerated everytime I use pipenv. I don't really need it since
I'm targeting old Python and Pip versions that even know about it. The
build system declared is setuptools which is what will be used anyway.
But I grow tired of always deleting the file (adding it to .gitignore
doesn't work because check-manifest complains). I _really_ hope it won't
break anything (if it will it will probably be in new Pip versions so I
will probably see it, I hope).