Drop support for Python 3.8, 3.9

[dstansby]

As per SPEC 0, don't need to support Python 3.8 or 3.9 any more.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.76%. Comparing base (b5f8506) to head (7b43985).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #381      +/-   ##
==========================================
- Coverage   85.82%   85.76%   -0.07%     
==========================================
  Files          13       13              
  Lines        1538     1531       -7     
==========================================
- Hits         1320     1313       -7     
  Misses        218      218              

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[joshmoore]

Thanks, @dstansby. I've recently heard of folks having issues on various platforms so let's see if any of that surfaces here. If not, I can definitely see getting this into the next largish bump.

[will-moore]

I was having issues getting Ice bindings installed on python 3.10 (our docs state that only python 3.9 was supported). In the end we found a work-around but for users who have working python 3.9 environments it could cause inconvenience to drop python 3.9 support here before we need to.
Happy to drop 3.8 support, but dropping 3.9 support feels a little early (unless supporting it is a burden to us that we are keen to drop?)

[dstansby]

The motivation here was zarr follows SPEC 0, so won't (in theory) be supporting Python 3.8/3.9 now anyway, so it makes sense to mirror that here? Happy to change this to just drop 3.8 though.