[PEP 517] Relax installation-time "pep517<0.7.0" requirement

[leycec]

python-for-android currently requires obsolete versions of the pep517 Python package at setuptools-based installation time via a pep517<0.7.0 requirement in the top-level setup.py script. This is bad for any number of obvious reasons, including:

• pep517 0.6.0, the most recent version of this package to satisfy this constraint, is nearly three years old. In Python years, pep517 is the geriatric centenarian with insufferable incontinence that habitually screeches "Gituffmuhlaaaaawn!" while throwing mouldy wonder bread at passing codebases.
• pep517 0.6.0 is so old that it's only tested to work under Python 3.4–3.7. Of those, only Python 3.7 has yet to hit its official End-of-Life (EOL) – but even that will happen shortly.
• pep517 0.6.0 is no longer packaged by any system package managers (e.g., Debian's apt, Arch's pacman, Gentoo's emerge). It's unclear whether pep517 0.6.0 even can be packaged by any system package managers or run under actively developed Python versions. By transitivity, python-for-android itself can no longer be reasonably packaged for any Linux distros.

In short, the pep517<0.7.0 constraint needs to be relaxed to cover the most recent stable release of pep517. In theory, this should be trivial; the p4a codebase only internally imports from pep517 twice in the python-for-android.pythonpackage submodule.

Thanks as always for all the phenomenal volunteerism, p4a team! You make the impossible at least slightly possible. :cowboy_hat_face: