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

[Drew Parsons]

Source: numba
Followup-For: Bug #972246
Control: severity -1 important

>From 0.51.2-2 numba is configured to build python3.8 only
(small patch, easily reverted once ready for python3.9).

>From 0.51.2-6 all debci tests pass (with python3.8), including armhf
(after skipping test_immutability_in_array_dispatch on armhf).

Hence downgrading bug severity from Serious to Important for now.