python3-pyyaml: make compatible with cython 3.x

python3-pyyaml: make compatible with cython 3.x

This has been rejected by upstream in favour of requiring obsolete cython
until there's 'proper' 3.x support. Months later, there's still no progress
so let's just take the rejected fix, as it does work (as reported by others as well),
and allows moving forward with cython.

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>