Type `Buffer`'s `flags` argument

[jakirkham]

Ensure the flags argument in Buffer is correctly typed so it can be passed verbatim to PyObject_GetBuffer. This can also avoid needless conversions to and from Python object type.

TODO:

• [ ] Unit tests and/or doctests in docstrings
• [ ] Tests pass locally
• [ ] Docstrings and API docs for any new/modified user-facing classes and functions
• [ ] Changes documented in docs/release.rst
• [ ] Docs build locally
• [ ] GitHub Actions CI passes
• [ ] Test coverage to 100% (Codecov passes)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 99.88%. Comparing base (145f57c) to head (c1c95fe). Report is 4 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #648 +/- ## ======================================= Coverage 99.88% 99.88% ======================================= Files 62 62 Lines 2723 2723 ======================================= Hits 2720 2720 Misses 3 3 ```

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[jakirkham]

Thanks David! 🙏