pip install ujson

Fightt hrow errors like this:

  Running setup.py install for ujson
    building 'ujson' extension
    arm-linux-gnueabihf-gcc -pthread -DNDEBUG -g -fwrapv -O2 -Wall -Wstrict-prototypes -fno-strict-aliasing -D_FORTIFY_SOURCE=2 -g -fstack-protector-strong -Wformat -Werror=format-security -fPIC -I./python -I./lib -I/usr/include/python2.7 -c ./python/ujson.c -o build/temp.linux-armv7l-2.7/./python/ujson.o -D_GNU_SOURCE
    In file included from ./python/ujson.c:39:0:
    ./python/py_defines.h:39:20: fatal error: Python.h: No such file or directory
     #include <Python.h>
                        ^
    compilation terminated.
    error: command 'arm-linux-gnueabihf-gcc' failed with exit status 1

In this case, it's because you also need python-dev package.

sudo apt-get install python-dev