Update pytest-cov in requirements-dev.txt from 2.1.0 to 2.5.1

[dependencies]

Dependencies.io has updated pytest-cov (a pypi dependency in requirements-dev.txt) from 2.1.0 to 2.5.1.

2.5.1

• Fixed xdist breakage (regression in 2.5.0). Fixes
  #157.
• Allow setting custom data_file name in .coveragerc. Fixes
  #145.
  Contributed by Jannis Leidel & Ionel Cristian Mărieș in
  #156.

2.5.0

• Always show a summary when --cov-fail-under is used. Contributed
  by Francis Niu in
  PR#141.
• Added --cov-branch option. Fixes
  #85.
• Improve exception handling in subprocess setup. Fixes
  #144.
• Fixed handling when --cov is used multiple times. Fixes
  #151.

2.4.0

• Added a "disarm" option: --no-cov. It will disable coverage
  measurements. Contributed by Zoltan Kozma in
  PR#135.

  WARNING: Do not put this in your configuration files, it's meant
  to be an one-off for situations where you want to disable coverage
  from command line.

• Fixed broken exception handling on .pth file. See
  #136.

2.3.1

• Fixed regression causing spurious errors when xdist was used. See
  #124.
• Fixed DeprecationWarning about incorrect addoption use. Contributed
  by Florian Bruhin in
  PR#127.
• Fixed deprecated use of funcarg fixture API. Contributed by Daniel
  Hahler in
  PR#125.

2.3.0

• Add support for specifying output location for html, xml, and
  annotate report. Contributed by Patrick Lannigan in
  PR#113.
• Fix bug hiding test failure when cov-fail-under failed.
• For coverage >= 4.0, match the default behaviour of coverage report
  and error if coverage fails to find the source instead of just
  printing a warning. Contributed by David Szotten in
  PR#116.
• Fixed bug occurred when bare --cov parameter was used with xdist.
  Contributed by Michael Elovskikh in
  PR#120.
• Add support for skip_covered and added
  --cov-report=term-skip-covered command line options. Contributed
  by Saurabh Kumar in
  PR#115.

2.2.1

• Fixed incorrect merging of coverage data when xdist was used and
  coverage was >= 4.0.

2.2.0

• Added support for changing working directory in tests. Previously
  changing working directory would disable coverage measurements in
  suprocesses.
• Fixed broken handling for --cov-report=annotate.