scripts: add make_bugs_pickle.py

This is a simple script which snapshots open bugs in the
zephyrproject-rtos/zephyr repository using the GitHub REST API.

It relies on the 'github' module, which is a de-facto standard for
accessing this API in Python:

  https://pygithub.readthedocs.io

We are already using this package in scripts/release/bug_bash.py, so
this is not a new dependency, even though it's a third party package
not available in the standard library.

The resulting bugs are stored in the standard library's 'pickle'
format, as a list of github.Issue.Issue objects. For more on pickle,
see:

  https://docs.python.org/3/library/pickle.html



I am choosing pickle because it is standard, easy to use, and we are
already using it in the build system to store the edtlib.EDT object
created by gen_defines.py. This is also therefore not a new
dependency (and even if it were, it's in the standard library).

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>