Drop el8, case-insensitive context, adjust decision callback

Highlights

• Case-insensitive context matching
• Drop support for el-8 and python-3.6
• Implement custom decision_callback in adjust()

Full Changelog

• Allow case-insensitive context matching
• Move the copr repository to the teemtee group
• Fix missing file require for unit tests
• Add callback to adjust() to make it observable by callers
• Drop support for el-8 and python-3.6
• Fix docs building on readthedocs
• Install yq needed for tmt integration tests
• Include srpm_build_deps in the packit config
• Remove metadata from Packit config
• Add missing build job to Packit config
• Update pre-commit checks to the latest versions
• Require fmf in tests, remove obsolete coverage
• Fix a typo in the merging minus sign documentation