Handle more optional modules for OpenWrt

A major issue is that the `json` module is broken for `python-light` on OpenWrt. The module won't work without another system package called `python-codecs`.

As an alternative Python package, `python-base` on OpenWrt provides a much smaller subset of the Python standard library than `python-light`. Modules like `atexit`, `tarfile`, `time`, `binascii`, and even `__future__` are all unavailable.

These modules are all imported without checking their existence.