Bump minimum numba version to 0.52

Created by: PhilipVinc

This is a fairly recent version (from this summer), but it's the only way to avoid numba#6041, where a more recent numpy with an older numba would give rise to errors.

Numba 0.52 is the version with the bugfix that works with any numpy.

it's not our fault, but since I know @gcarleo appreciates things just working...