Fix syntax of `tests_require` option
Fix syntax of `tests_require` option

Confusingly, the option has no trailing 's' like `install_requires`.

See:

    https://pythonhosted.org/setuptools/setuptools.html
    https://travis-ci.org/pytest-dev/pytest-qt/jobs/100969573#L363

pytest-dev/pytest-qt,The-Compiler/pytest-qt