Fix packaging checks and simplify CI

[janbrasna]

Due to a bug in twine<6.0.1 not reading the correct metadata using wildcards thus failing the tox -e packaging steps, I've moved the pin from 5.1.x to 6.1.x to include some additional compatibility with packaging dependencies.

This also bumps outdated actions/* and generally simplifies and speeds up the workflow.

Inlining the h2spec step also allows for getting rid of its install script and exposing the access token.

NB: To currently pass ruff/mypy checks, I had to fix runtime-cast-value (TC006) by quoting the type expressions in h2/stream occurrences of cast() …

Logs: actions/runs/14669502174

[janbrasna]

Hmmh this was perhaps not exactly fortunate, as the id seems to be defined in repository branch protection settings as required checks… in that case feel free to revert this back:

Suggested change

	test:
	tox: