legacy dependencies

[bmcfee]

This PR is similar to #145, but implements the alternative strategy described in #144 of making deprecated dependencies optional on python 3.13.

This isn't necessarily meant to supersede #145, just to provide an alternative implementation that's ready to go in case @sampsyo wants to go this route instead.

Summary of changes:

• Added audioop-lts as a hard requirement on python >= 3.13. This is necessary for the lin2lin function used in all rawread paths.
• Reorganized the codec ordering in RawRead so that wave (still supported) is first, followed by aiff and au.
• Implemented soft failure for aifc and sunau modules, issuing a warning if the module is missing.
• Added legacy mode installation which includes dependencies for standard-aifc and standard-sunau if python >= 3.13. Can be invoked by python -m pip install audioread[legacy].
• Updated documentation (README) to reflect the status of deprecated packages and legacy install instructions.

[lucas42]

Cool. I didn't even realise pip had an option to do this!
Can I suggest adding python 3.12 and 3.13 to the test matrix in .github/workflows/main.yml?

[bmcfee]

Can I suggest adding python 3.12 and 3.13 to the test matrix in .github/workflows/main.yml?

I don't think that would do anything at present, since the workflow doesn't actually run pytest, or even install the package. It also appears that the CI action has not run for 2 years at this point - probably that maintenance should happen in a separate PR.

[lucas42]

Fair enough. I managed to get the tests running on my fork, but never really dug into what they were actually doing.