Refactor `stim_pybind::PyPaulString` -> `stim::FlexPauliString`

[Strilanc]

• Also fix sinter.Task.strong_id not correctly checking for circuit not being set
• Fix stim.PauliString.__init__ specifying pybind11 overloads instead of manually doing resolution.
  • This notably improved the documentation of the method.
  • Force positional arg on the init argument, but silently maintain backwards compatibility with old keyword
• Fix stim.PauliString.from_unitary_matrix signature typo float -> complex
• Fix stim.PauliString.before signature typo after -> before