[Merge] ~racb/ubuntu-sponsoring:cache-redundant-api-calls into ubuntu-sponsoring:main

[Benjamin Drung]

Review: Approve

I tested it locally. It cuts down the execution time from 15 to 8 minutes. The resulting json files are identical.

Merged with following changes:
* added python3-cachetools to README.md
* sort setup.py requires
* format code with `black -C`
* run isort
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https://code.launchpad.net/~racb/ubuntu-sponsoring/+git/ubuntu-sponsoring/+merge/468585
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