[ENH] Reproducible output

[da-woods]

Describe your issue

This is just a very broad issue to collate what we might do if we want Cython to give reproducible output:

• It looks like Cython ends up embedding full paths into the source code [ENH] normalize build path #5949. Ideally this'd apply to both c code and comments embedded into the c source.
• Do we ever embed a date/time into the output? (I don't think so but could be wrong).
• Anywhere where sets are iterated over and the order is important - e.g. [BUG] Non-deterministic output for local vars with multiple types #5986
• dict in older versions of Python (I'm inclined to treat this as a lost cause)
• I think there may be places where the globally defined BuiltinScope, CythonScope or their entries are modified as Cython works. This means that if multiple modules are compiled in parallel then the exact output may depends on the order of parallel execution.
• Other parallelization issues? (e.g. there used to be an issue with redirecting error output that was fixed in f372c5a, but it likely isn't the only such issue)
• ????

There's then the question of how best to test it.