[py-dev] pytest-cov

[meme dough]

Hi,

The --cov option is now filesystem paths as that should be how it will
work with the upcoming source option from coverage.  Help text
updated.

Warning when nothing to report on.

Reports on generation of html, xml and annotated source.

Minimised imports.

Haven't moved DistMaster / DistSlave yet as found easier for
minimising imports if not in pytest-cov, may make a separate module
there, will consider further.

:)