package/python-uvloop: force re-cythonize on build

The generated loop.c provided with the tarball was generated with
Cython 0.29.10 and it doesn't compile against the new Python 3.8
headers. Upstream is now working on a new uvloop release which will
use the newest Cython (0.29.13).  Until then, we will remove the
current loop.c and force regenration of it.

Fixes:
 - http://autobuild.buildroot.net/results/bffe1b2ee7b674f870e15398680ce5b5b22fd296



Signed-off-by: Asaf Kahlon <asafka7@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>