Dump pep425tags

Hi,

As I ran into trouble building wheels on Windows (see #20 ), I have rewritten things a bit to ensure that things are more in line with what wheel does, following their move to the packaging.

Testing this has been a pain, as I cannot reference git checkouts in build-system.requires, and had to test this using a conda build of this branch in Github Actions, and things seem to work fine for macOS and Linux, but haven't been able to test this on Windows due to meson not installing nicely into conda environments without additional options passed to meson...

I was wondering if you could merge this in, and make a beta release on PyPI that I could reference? Let me know if you need more changes first!

Thanks in advance,

Tom