Add `incomplete_results` property to PaginatedList (#3396)

The patch to fix #2166, fix #2253, and fix #2812

According to
https://docs.github.com/en/rest/search/search?apiVersion=2022-11-28#timeouts-and-incomplete-results,
the incomplete results are expected and should be identified by the
`incomplete_results` flag.

This patch adds the flag and tests for it.

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Co-authored-by: Enrico Minack <github@enrico.minack.dev>