## Summary
As discussed in https://github.com/astral-sh/ruff/issues/16983 and
"mitigate" said issue for the alpha.
This PR changes the default for `PythonPlatform` to be the current
platform rather than `all`.
I'm not sure if we should be as sophisticated as supporting `ios` and
`android` as defaults but it was easy...
## Test Plan
Updated Markdown tests.
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Co-authored-by: David Peter <mail@david-peter.de>