ci: add Python 3.12

[woodruffw]

Chore; adds 3.12 to our test matrix.

[woodruffw]

. env/bin/activate && \
	 pytest --cov-append --cov=sigstore test/unit --skip-online -vv --showlocals && \
	python -m coverage report -m 
ImportError while loading conftest '/home/runner/work/sigstore-python/sigstore-python/test/unit/conftest.py'.
test/unit/conftest.py:32: in <module>
    from sigstore_protobuf_specs.dev.sigstore.bundle.v1 import Bundle
env/lib/python3.12/site-packages/sigstore_protobuf_specs/dev/sigstore/bundle/v1/__init__.py:7: in <module>
    import betterproto
env/lib/python3.12/site-packages/betterproto/__init__.py:39: in <module>
    from ._version import __version__
env/lib/python3.12/site-packages/betterproto/_version.py:1: in <module>
    from pkg_resources import get_distribution
E   ModuleNotFoundError: No module named 'pkg_resources'
make: *** [Makefile:82: test] Error 4

From a quick look, it looks like betterproto depends on pkg_resources, and we depend on betterproto via sigstore-protobuf-specs.

Looks like a fix was released with 2.0.0b6: danielgtaylor/python-betterproto#462

Will get this fixed in sigstore-protobuf-specs.

[woodruffw]

Upstream fix: sigstore/protobuf-specs#151

[woodruffw]

This should be good to go now -- sigstore-protobuf-specs has been bumped and now works correctly with 3.12.