RFC: Python 3.7 package resources support

[shizmob]

This series of patches adds support for loading web resources through Python's importlib.resources abstraction, allowing them to be packed into whatever module loading mechanism is used (e.g. zipimport). This would allow distributing copyparty as a single .py without needing any kind of file system access in /tmp or elsewhere.

Three pending improvements that make it RFC for now:

• Make package name used for access configurable through E instead of hardcoded in util.py
• Verify walk_resources() correctness
• Add support for the older pkg_resources API

[shizmob]

Motivating example: attached is a version of copyparty (tar'd because GitHub, from the HEAD of my branch at time of writing, 2997c42) packed with a simple packer I wrote into a single .py file that doesn't need file system access other than for serving files.

copyparty.py.tar.gz

[9001]

Nice! This will be great for the pyz / zipapp too, since that currently extracts the webdeps and jinja-templates into a temporary folder, which was somewhat problematic due to systemd's tmpfiles cleaner.

Thanks for the cool stuff 😁