Motivation for this change

scikit-build appears to have broken when pytest was bumped to 4.x. However the only thing complaining about the presence of pytest4 was one of its dependencies, pytest-virtualenv. So what I did instead was bump that along with pytest-shutil, pytest-fixture-config and pytest-server-fixtures because they all actually come from the same "master package" https://github.com/manahl/pytest-plugins and all have a recent 1.7.0 release which makes them compatible with pytest4 and allows me to remove any pytest_3 requirement for them.

Finally I had to disable test_source_distribution in scikit-build to get its tests to pass. Previously I think it had actually been auto-skipping this test so I don't think I'm actually net disabling anything.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nix-review --run "nix-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Assured whether relevant documentation is up to date
• Fits CONTRIBUTING.md.