Bump minimum C++ version to 17 on macOS

[clin1234]

Is there a particular reason to keep clang++ to C++11 on macOS? Because according to https://github.com/actions/runner-images/blob/macos-14-arm64/20251120.0014/images/macos/macos-15-arm64-Readme.md#language-and-runtime, the macOS image on GH comes with LLVM 17, and also according to https://clang.llvm.org/cxx_status.html, clang++ defaults to C++17 as of LLVM 16.

The reason that I'm asking is to attempt to introduce constexpr

[jamadden]

We need to maintain compatibility with the oldest compilers that CPython supports for the versions we support (3.10+), and they're currently on C11. I know we get users from a wide variety of platforms with a wide variety of compilers, so we don't want to cut them off unnecessarily. (For example, I know we still have user(s) on macOS 10.6 with GCC 14.)

[clin1234]

We need to maintain compatibility with the oldest compilers that CPython supports for the versions we support (3.10+), and they're currently on C11. I know we get users from a wide variety of platforms with a wide variety of compilers, so we don't want to cut them off unnecessarily. (For example, I know we still have user(s) on macOS 10.6 with GCC 14.)

On which platforms and compiler toolchains is C++11 the default for the supported Python versions?

[jamadden]

Read what I wrote. I said C11. CPython adds -std=c11 to the compiler options.

[clin1234]

I was referring to this in particular: https://github.com/python-greenlet/greenlet/blob/master/setup.py#L67