Update test matrix and add scheduled tests

[matt-graham]

Updates test matrix to run tests across all supported Python versions (currently Python 3.8 – 3.12) ~~and operating systems~~. Also adds weekly scheduled test runs to resolve #208.

Given this makes the test matrix quite large ~~(15 combinations of OS and Python version)~~, and Python 3.8 and Python 3.9 are both out of the recommended support window of both SPEC0 and NEP29, I would also recommend we consider dropping Python 3.8 and Python 3.9 support (in a separate PR).

EDIT: Appears that there are not Windows wheels available for healpy / pyssht and we're getting a segfault somewhere when using MacOS / Python 3.9+ so reverting to only testing on Ubuntu + (MacOS, Python 3.8) for now.