build: drop Travis in favor of Actions

GitHub Actions is running all tests already present on Travis, as well
as building on more platforms (OS X and Windows). With Travis we're also
getting timeouts more frequently than with Actions, which gives the
false impression tests are failing (making it harder to triage PRs ready
to merge).

To make our config simpler, CI.yml and pythonpackage.yml got merged. The
coverage is also increased by running tests on OS X.

Signed-off-by: Matheus Marchini <mmarchini@netflix.com>

PR-URL: https://github.com/nodejs/node/pull/32450


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Christian Clauss <cclauss@me.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ben Coe <bencoe@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Michaël Zasso <targos@protonmail.com>