Bug#972246: numba ftbfs in unstable with python3.9 as supported python3 version

[Stefano Rivera]

Looks like it's going to take a while to get Python 3.9 support in the
numba stack:
https://github.com/numba/numba/issues/6345#issuecomment-706722002

This isn't just a little python library, it's a Python JIT for Python,
so I'd believe that supporting 3.9 is non-trivial.

SR

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Stefano Rivera
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