Consider making 3.7 compatible

[DangerMouseB]

c

jones uses a couple of c api calls that are in 3.9 but not in 3.7 - if we backtrack to 3.7 we might also do a forward track to 3.11 as it is faster - don't really want to maintain parallel versions but the speed is enticing and by the time it's done 3.11 will likely have more mainstream usage.

python

remove :=

is it important? is it worth it?

We need to have a way of organising the c code for multiple python versions (the := isn't particularly onerous to do)