* Restoring `const` removed from pytypes.h in PR #3254, adding tests reflective of user code that breaks when those `const` are removed.
* clang-tidy NOLINTs (and one collateral fix).
* Inserting PYBIND11_CONST_FOR_STRICT_PLATFORMS
* Trying `defined(__APPLE__)`
* Trying again: `auto it` for strict platforms.
* Adding NOLINTNEXTLINE(bugprone-macro-parentheses), expanding comments.
* Labeling all changes with `PR #3263`, for easy reference, and to make it easy to undo these changes if we decide to do so in the future.
