package/python-matplotlib: simplify version checks

Hopefully, this should fix the following error on one of the
autobuilders:

png: no  [The C/C++ header for libpng (png.h) could not
     be found.  You may need to install the development
     package.]

Fixes:
 - http://autobuild.buildroot.org/results/afddcc44b2fb7983244f24542bfae921869e4ab8



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>