package/python-numpy: bump to version 1.18.2

package/python-numpy: bump to version 1.18.2

Since version 1.18.0, Numpy only supports Python 3.x, so we make it
depend on BR2_PACKAGE_PYTHON3 and propagate this to its reverse
dependencies.

Also, since upstream commit d630d96598f1b7ea044d60bea8cd2b87778f072a,
the Cython generated files are no longer part of released tarballs,
and therefore Numpy requires python-cython on the host as a build
dependency.

Cc: Samuel Martin <s.martin49@gmail.com>
Cc: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Cc: Esben Haabendal <esben@geanix.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>