py-test-cov: updated to 2.6.0

2.6.0:
Dropped support for Python < 3.4, Pytest < 3.5 and Coverage < 4.4.
Fixed some documentation formatting. Contributed by Jean Jordaan and Julian.
Added an example with addopts in documentation.
Fixed TypeError: 'NoneType' object is not iterable in certain xdist configurations.
Added a no_cover marker and fixture.
Fixed broken no_cover check when running doctests.
Fixed various issues with path normalization in reports (when combining coverage data from parallel mode).
Report generation failures don窶冲 raise exceptions anymore. A warning will be logged instead.
Fixed multiprocessing issue on Windows (empty env vars are not passed).

(adam)