I'm very reluctant to do this as it will typically mean that newer CPython versions will now require a release with a changed build range anyway. I can certainly build gssapi on some Linux distros so it seems like a C99 compiler is available in some cases. We'll need to try and figure out when that has happened and whether it's wide spread enough to need something extra to constrain it or not.

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I'm trying out pinning a specific Cython version to enable more deterministic builds with #363. I may move back in the future if that turns out to have more overhead/maintenance needed but let's see how we go for now.

While this means that some older distro's might need extra flags to be set for things like the C99 compiler they can continue to use older releases or the workarounds needed. The aim here is for the latest to support the current Python and distro versions still in mainline support.