Ignore warnings building PyYAML wheels on 3.10.0b4

Recent versions of pip and wheel are raising DeprecationWarning when
trying to build wheels of PyYAML due to deprecated license_file
metadata, solved once https://github.com/yaml/pyyaml/pull/536 lands
and is included in a new release.

Recent Cython is raising a FutureWarning when trying to build wheels
of PyYAML due to a missing language_version option, solved once
https://github.com/yaml/pyyaml/pull/539 lands and is included in a
new release.