Pin PyYAML to avoid broken installation on Cython3

With Cython 3.0 release, PyYAML builds started crashing due to a incompatible setup script.

Mitigate by pining PyYAML to the recently updated HEAD of release/6.0, which defines the build requirement of Cython<3.

Whenever there's a new release of PyYAML this can be reverted.

Part of: #30 (closed)

Signed-off-by: Tales Aparecida tales.aparecida@redhat.com