Make coverage optional

The underlying purpose of this merge request is to allow running tests against an installed set of dependencies without any versions pinned.

Currently it is impossible to unpin the coverage dependency, as tests currently fail with coverage >= 4.4.2, as such this patch moves isolates coverage support to tests being run under tox, where we can control the dependency versions under test, and removes built-in coverage support for running tests against installed host dependencies where we cannot control the dependency versions.

Note that it is important to support running tests against host dependencies, as this is how distro packagers can test for problems running against a dependency chain controlled by the distro.

Additionally, this branch adds some new optionality in tox.ini, allowing invocation of the tests without coverage present at all.