* Fix bugs found by mypy
* Add some missing types and runtime checks, remove unused code
* Make ZenithPageserver start right away for better type safety
* Add `types-*` packages to Pipfile
* Pin mypy version and run it on CircleCI
* Add yapf run to CircleCI
* Pin yapf version
* Enable `SPLIT_ALL_TOP_LEVEL_COMMA_SEPARATED_VALUES` setting
* Reformat all existing code with slight manual adjustments
* test_runner/README: note that yapf is forced
It's not realistic to enable full-blown type checks
within test_runner's codebase, since the amount of
warnings revealed by mypy is overwhelming.
Tests are supposed to be easy to use, so we can't
cripple everybody's workflow for the sake of imaginary benefit.
Ultimately, the purpose of this attempt is three-fold:
* Facilitate code navigation when paired with python-language-server.
* Make method signatures apparent to a fellow programmer.
* Occasionally catch some obvious type errors.