Fix deprecation warning

[ammgws]

Noticed this when my distro's package manager was building python-mpv today:

==> Making package: python-mpv 1.0.1-1 (Thu 14 Jul 2022 06:38:46 PM JST) ==> Checking runtime dependencies... ==> Checking buildtime dependencies... ==> WARNING: Using existing $srcdir/ tree ==> Starting build()... /usr/lib/python3.10/site-packages/setuptools/dist.py:757: UserWarning: Usage of dash-separated 'description-file' will not be supported in future versions. Please use the underscore name 'description_file' instead

[jaseg]

Thanks!