package/python-pyudev: fix build with python 2

Build with python 2 is broken since bump to version 0.22.0 in commit
0adb141d:

error:   File "/usr/lib/python2.7/site-packages/pyudev/_ctypeslib/utils.py", line 54
    lib = cdll.LoadLibrary(f'lib{name}.so')
                                         ^
SyntaxError: invalid syntax

Fixes:
 - http://autobuild.buildroot.org/results/8b35ca6910dfd881953968f8d88ac842d57c9262



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>