rust-inner-revlog: derive Debug for IndexHeaderFlags

rust-inner-revlog: drop some outdated comment

rust-config: add more ways of reading the config These will be needed for future patches of this series to interpret more complex/different config values.

util: make buffer readonly There is no use of writable buffers anywhere in the code, and this helps us make sure we don't get into unsound territory when sharing memory with Rust. This `toreadonly` method was not available in Python 3.6, but we dropped the support for it earlier that week, so no need for any compatibility code.

setup: avoid the deprecated `distutils.spawn.find_executable` I noticed this was flagged with `DeprecationWarning` in py3.12 with `setuptools` 74.1.2, and it suggested `shutil.which()` instead. The signatures aren't the same, but the additional `mode` argument in the middle of the latter defaults to `os.F_OK | os.X_OK`, which maintains the same semantics.

setup: drop the hack to disable linker warning 4197 on Windows I don't see this when building on Windows with py3.8 or py3.12, so either the code was fixed, or (more likely) the compiler stopped warning about it some time after VS 2008. If we do have to put this back, it would probably be better to put a `#pragma` in a header file somewhere, and avoid `setuptools` technical debt.

ci: also offer to test 3.12 with rust The rust-cpython binding got 3.12 support very recently, it is worse keeping on a tighter watch.

ci: add the option to test more Python versions It seems like a good idea to be able to test the lowest version we support. And there have been enougth issue with 3.12 that we need to be able to make sur we do not break it. We should probably get a matrix setup for more version and flavor, but that is a simple and efficient start.