ci: scripts: Suppress pylint warnings for the sh library

pylint does not like how this library works and generates spurious
warnings like

    scripts/ci/get_modified_tests.py:55:13: E1121: Too many positional
    arguments for function call (too-many-function-args)

    scripts/ci/get_modified_tests.py:57:13: E1123: Unexpected keyword
    argument '_tty_out' in function call (unexpected-keyword-arg)

These warnings are useful enough to want to have enabled in the upcoming
pylint CI check, so suppress them here.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>