use json format for config file

[dt-rush]

This PR replaces the config file format with human-friendly JSON, and also adds code to seamlessly convert old configs to JSON if detected.

---

rationale

As the recent API key overload made clear (issue #551), users being able to edit the config themselves is an important feature (modifying the configured default API key is not really feasible by hand, as one user remarked).

The config is currently stored as a pickled binary file, which is not user-friendly.

Pickled binary storage is not necessary for any reasonable disk-space reasons. The difference in the old config style vs json is on the order of tens of bytes, as can be seen:

pi@raspberrypi:~/.config/mps-youtube $ python3

>>> import pickle
>>> import json
>>> with open('config', 'rb') as cf:
...     with open('config.json', 'w') as cfj:
...             json.dump(pickle.load(cf), cfj)
... 
>>> 
KeyboardInterrupt

pi@raspberrypi:~/.config/mps-youtube $ ls -l
total 20
-rw-r--r-- 1 pi pi  708 Jul 15 03:14 config
-rw-r--r-- 1 pi pi  676 Jul 15 03:47 config.json

Signed-off-by: dt-rush nickp@balena.io

[ids1024]

Thanks! The set command exists to modify configuration, and I don't see a strongly compelling reason for the format to be text based. But generally text based configuration formats are nicer to have, and there isn't much reason not to.

[TsukiZombina]

Thanks! The set command exists to modify configuration, and I don't see a strongly compelling reason for the format to be text based. But generally text based configuration formats are nicer to have, and there isn't much reason not to.

The set command doesn't work for me, just changing configuration file, as I mention in #551.