tests: Use PyVirtualDisplay instead of xvfbwrapper

[mgorny]

Changes proposed in this PR

Use the more modern PyVirtualDisplay package instead of xvfbwrapper to run Xvfb. Most importantly, it uses the more robust approach of starting Xvfb with -displayfd and it is actively maintained upstream. Unfortunately, this does not seem sufficient to entirely eliminate random test failures.

It is very important to keep up to date tests and documentation.

• [ ] Tests added/updated
• [ ] Documentation updated

Is your code right?

• [x] PEP8 compliant
• [x] flake8 passed

[mgorny]

Well, let's see how it fares on CI. Sometimes I still get a single test failure, at other times all pass. Perhaps the only way to really avoid that is to use something like pytest-rerunfailures to retry 2 or 3 times on failure.

[mgorny]

Thanks a lot!

[BoboTiG]

Thanks to you :)