revive pytest-html maintenance

pytest-html needs an active pool of maintainers as it slowly degraded.

I am offering to help with most maintenance tasks and especially around making/keeping CI/CD green and reviewing upcoming contributions.

pytest-html is a key ingredient in testing implementation we use on several openstack projects, also directly used with my pytest-molecule plugin, so I have a strong interest in keep in it working.

I hope others will also be interested to join this, as it much better to avoid me becoming another single point of failure.

One of the first things I want to do is to migrate testing to Github Actions as it far more reliable and flexible than Travis, and assure we have a devel job with tests with unreleased versions of major dependencies, like pytest (so we avoid surprises when they release). like we had with pytest-6.0.0. Also Github Actions can take care of release process in a safe way, which involve using a bot account to publish releases.

PS. I do not yet have the rights on the project.

No files or tests are named. Start by reviewing the existing Travis-based testing and release setup, then assess the proposed GitHub Actions migration and a development job using unreleased major dependencies. Done means the CI/CD and release process are migrated, dependency coverage is defined, and maintenance responsibilities are shared.