Use new WeasyPrint URL fetcher

[liZe]

This comes with a few minor changes:

• make_url_fetcher’s next_fetcher should now be a URLFetcher,
• bytestring URLs are now forbidden,
• the fetcher’s response is a URLFetcherResponse,
• drop Python 3.10 support, as 308 redirects used by recent versions of Flask are supported in 3.11+.

[frenzymadness]

The diff looks good to me, and I've used one of my big projects to test this, and it works well (the current version generated ~350 warnings).

I've just found one nitpick in the Readme.