MNT: fix compatibility with Python 3.14

[tacaswell]

The ast.Str class was deprecated in 3.8 and will be removed in 3.14

[takluyver]

Thanks, nice catch! flit_core still supports Python down to 3.6, so could I ask you to do a little bit of compatibility code here? Something simple like if sys.version_info >= (3, 8) should be fine.

I know I'm pretty weirdly conservative on this. In most projects I'd have dropped such old Python versions some time ago. But flit_core is kind of in a special position, being used to build a whole lot of other packages, including key packaging tools like build & installer. Python 3.6 is still accounting for 2-3% of downloads from PyPI, so I'd rather keep it compatible than have some users getting an older version of flit_core.

[takluyver]

Thanks Tom!

[AA-Turner]

Sorry to be one of those people, but please would it be possible to have a release with this commit? I'm currently unable to test with Python 3.14 in CI as pip is failing with this error.

A