build: support Python 3.13

[shiftinv]

Summary

This adds the 3.13 package classifier, and updates dependencies where needed (see below). Unlike 3.12, 3.13 doesn't really require a whole lot of code changes.

CI

This does not add 3.13 to the CI matrix, as some dependency updates, namely pycares (through aiodns) and libcst, have already dropped Python 3.8 support in the versions that add prebuilt 3.13 wheels, which results in PDM refusing to resolve dependencies.
While the current dependency versions technically all run fine on 3.13, the lack of prebuilt wheels means each CI job would spend a solid minute just building libcst and pycares, which should definitely not be necessary.
Once we update PDM, https://pdm-project.org/en/latest/usage/lock-targets/ can likely solve this.

audioop

3.13 removed audioop, which is used in PCMVolumeTransformer.
Re-implementing this in pure Python is certainly doable, but given that it's somewhat performance-critical and ultimately just number crunching, even an optimized implementation is not particularly fast compared to the native implementation.
To put this into perspective, have some (rough) pyperf benchmarks of a couple different attempts:

native: Mean +- std dev: 9.28 us +- 0.36 us
new_thingy: Mean +- std dev: 352 us +- 4 us
new_thingy_array: Mean +- std dev: 259 us +- 4 us
new_thingy_dict_lookup: Mean +- std dev: 232 us +- 6 us
new_thingy_array_lookup: Mean +- std dev: 124 us +- 2 us
new_thingy_bytes_lookup: Mean +- std dev: 83.6 us +- 1.8 us

`bytes_lookup`

shortstruct = struct.Struct("<h")  # signed short LE

# precomputed lookup table for one specific factor
bytes_lookup = []
for i in range(65536):
    v = int(factor * (i - 32768))
    v = 32767 if v > 32767 else (-32768 if v < -32768 else v)
    bytes_lookup.append(shortstruct.pack(v))

def new_thingy_bytes_lookup(fragment: bytes, width: int, factor: float):
    a = array.array("h", fragment)
    return b"".join([bytes_lookup[s + 32768] for s in a])

Long story short, disnake[voice] now adds audioop-lts on 3.13, see 11e87cc for rationale. In the long term, I would prefer getting rid of audioop entirely by no longer shipping PCMVolumeTransformer in the library.

Checklist

• If code changes were made, then they have been tested
  • I have updated the documentation to reflect the changes
  • I have formatted the code properly by running pdm lint
  • I have type-checked the code by running pdm pyright
• This PR fixes an issue
• This PR adds something new (e.g. new method or parameters)
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)