Summary

As part of Gentoo packaging, we are running matplotlib's test suite. While doing that, I'm frequently seeing test failures due to "images not being close enough". Interestingly, the RMS I'm seeing often fits in the range provided for the "non-x86_64" code path, even though I'm running the tests on x86_64. This leads me to believe that the problem is not really "x86_64 vs. other platforms", but "specific x86_64 CI setup".

Proposed fix

1. Replace the inline platform.machine() == 'x86_64' checks repeated all over the tests with a shared constant in a single file.
2. Either make it possible to override it via an environment variable (i.e. we'd use the variable to allow higher RMS when testing), or actually default to higher RMS and enable lower RMS when using a variable specific to CI testing (e.g. MPL_X86_64_CI or something like that).

If you agree, I can make a pull request — just let me know which solution you prefer.