ci: Add packit packaging

[LecrisUT]

Description

This adds some automation for packaging and testing Fedora releases. A few benefits this brings:

• Quicker updates for Fedora releases. This automation makes PRs to https://src.fedoraproject.org/rpms/pybind11 when a new release is created
• More testing on various Fedora releases and its package versions. Can also try to add EPEL for centos/rhel
• Get notified if upcoming changes in rawhide are affecting the project
• More seamless collaboration between downstream and upstream. E.g. we don't have to wait on a new release to suggest patches.
• Reverse dependency tests. scikit-build-core has a packit workflow, which in principle could be triggered from builds here.

An example of the builds being run: https://copr.fedorainfracloud.org/coprs/g/scikit-build/pybind11/build/7276661/

TODO:

• [ ] Add github app to enable the CI: https://packit.dev/docs/guide#1-set-up-packit-integration
  • It will ask for linking the github owner's account to a fedoraproject account, but we can overwrite it manually on packit side and link it to my account
• [x] Configure and modernize the build process
  • I have considered using the downstream spec file, but there are a few points I want to address there as well, so I want to investigate them fresh over here
  • More comments in #5095