av riscv64 support

[luhenry]

Package name (on PyPI)

av

Source repository

https://github.com/PyAV-Org/PyAV

Version(s) requested

18.1.0

Why is this needed?

riscv64 wheel build added in #461

Additional context

Wheel bundles prebuilt FFmpeg (pyav-ffmpeg): GPL-2.0-or-later libx264/libx265 plus LGPL FFmpeg, GnuTLS, Nettle, GMP, libunistring, alsa-lib and LAME. Licence texts are packaged and the corresponding sources published as a release asset.