Build wheels for Python 3.15

[p-r-a-v-i-n]

Changes

Fixes #344

cbor2 already declares and tests support for Python 3.15, but release 6.1.4 does not contain CPython 3.15 wheels because the publishing workflow uses cibuildwheel 4.1.
cibuildwheel 4.2 enables CPython 3.15 builds by default using the ABI-stable Python 3.15 RC. This update will make the next cbor2 release publish cp315 wheels

Refs docs:

• https://github.com/pypa/cibuildwheel/releases/tag/v4.2.0

Checklist

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) which would fail without your patch
• You've updated the documentation (in docs/), in case of behavior changes or new
  features
• You've added a new changelog entry (in docs/versionhistory.rst).

[coveralls]

coverage: 95.016%. remained the same — p-r-a-v-i-n:build-python-315-wheels into agronholm:master

[agronholm]

I will publish new wheels with the new release. Don't rush me.

[agronholm]

FYI, wheels for 3.15 and 3.15t are on PyPI now.