Add rerun capability to pytest test failures

I've noticed with a recent PR (https://github.com/nexB/scancode-toolkit/pull/2958) that one test in particular (`FAILED tests/scancode/test_cli.py::test_scan_does_scan_php_composer`) seems to fail with near-regularity, yet often will subsequently pass after being manually rerun.

As @pombredanne pointed out when we were discussing how best to handle this behavior, https://github.com/pytest-dev/pytest-rerunfailures may offer an effective method to address these sorts of intermittent failures. Fingers crossed. ;-]