ci: Add support to test Python 3.15.0

[pvital]

This PR add a new GitHub Actions to test Instana Tracer against Python 3.15.0, still in development.

The Action stores the output of the pytest command as a run artifact and it is configured to run:

1. Manually.
2. Every Monday to Friday at 3:01 AM.
3. On every push on main branch.

With that, we can always test the next Python version and not block the PR’s and development of Instana Tracer. The execution of this GH Action can be checked here.

The decision to use GH Action instead of the already used CircleCI, is because CircleCI was hanging due to no interaction with some unit tests, and the pytest-timeout was not taking effect there. GH Action had a better result and output for this case.

[CagriYonca]

It seems the Slack notification job is failing due to the issue below:

Run python .github/scripts/announce_pr_on_slack.py
❌ SLACK_TOKEN environment variable is required
Error: Process completed with exit code 1.

[CagriYonca]

Looks good to me

[GSVarsha]

Looks good to me!