Update pytest-timeout in requirements-dev.txt from 1.2.1 to 1.3.1

[ghost]

Dependencies.io has updated pytest-timeout (a pypi dependency in requirements-dev.txt) from "1.2.1" to "1.3.1".

1.3.1

• Fix deprecation warning on Python 3.6. Thanks Mickaël Schoentgen
• Create a valid tag for the release. Somehow this didn't happen for
  1.3.0, that tag points to a non-existing commit.