support/testing: tyest_python_pybind.py: fix flake8 errors

support/testing/tests/package/test_python_pybind.py:1:1: F401 'os' imported but unused
support/testing/tests/package/test_python_pybind.py:3:1: F401 'subprocess' imported but unused
support/testing/tests/package/test_python_pybind.py:6:1: E302 expected 2 blank lines, found 1
1     E302 expected 2 blank lines, found 1
2     F401 'os' imported but unused

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>