Merge #21096: Re-add dead code detection

3f8776a1391c3978ed66144df15fd9bcb9edd35d Re-add dead code detection (flack)

Pull request description:

  This re-adds unreachable code detection for Python based on `vulture`.

  Effectively, this reverts f4beb4996d27f2cdaf4f0a63e7dc044bf17decce. The difference to the previous version is that this runs with the `--min-confidence 100` setting. From https://pypi.org/project/vulture/:

  > Use `--min-confidence 100` to only report code that is guaranteed to be unused within the analyzed files.

  So this should avoid the previous issues where static analysis had wrong positives due to the dynamic nature of Python code by only reporting things that are unambiguous (such as code after a `return` statement). As such, there is not suppressions list.

  My motivation was mainly #21081 which would have been caught by this (as can be seen by the CI run failing). This is still marked as draft because #21081 is needed to get the linter to pass. Also, there is a second problem that this found (see https://github.com/bitcoin/bitcoin/pull/19509/files#r571454691). From what I can tell, this is a spurious type comment that could just be removed (or if that line has no side effects it could also be deleted altogether?). I could add a commit here to fix it, but I wanted to see if there is interest in having this linter again in the first place

ACKs for top commit:
  practicalswift:
    ACK 3f8776a1391c3978ed66144df15fd9bcb9edd35d

Tree-SHA512: 52314ad4f627d969de1eb15375ca677ed86a2e816fe773756a1ce22421214ba407b5a09a4bf701a3aab1a10c7b336f548e4cef3327edf154acba55e987db21f6

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