Remove Python dependency

[rieck-srlabs]

We currently use a Python script to generate and set new IMEIs.

Our dependency on Python bloats the package and increases execution times. There is also a rare issue where the Python script fails to set an updated IMEI. Our current understanding is that this is due to our exclusive usage of the serial interface to the modem, which might break when the modem is used by other parts of the system at the same time.

In the future, we can migrate to Lua, which is natively supported by OpenWRT. Most of the actual work is already completed in luhn.lua.