Update pytest-cov in requirements.txt from 2.4.0 to 2.6.0

[dependencies]

Dependencies.io has updated pytest-cov (a pypi dependency in requirements.txt) from "2.4.0" 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. Contributed by
  Samuel Giffard in
  #195.
• Fixed TypeError: 'NoneType' object is not iterable in certain
  xdist configurations. Contributed by Jeremy Bowman in
  #213.
• Added a no_cover marker and fixture. Fixes
  #78.
• Fixed broken no_cover check when running doctests. Contributed by
  Terence Honles in
  #200.
• Fixed various issues with path normalization in reports (when
  combining coverage data from parallel mode). Fixes
  #130.
  Contributed by Ryan Hiebert & Ionel Cristian Mărieș in
  #178.
• Report generation failures don't raise exceptions anymore. A
  warning will be logged instead. Fixes
  #161.
• Fixed multiprocessing issue on Windows (empty env vars are not
  passed). Fixes
  #165.