Fixed previous commit to work on non-Windows where os.altsep is None
Fixed previous commit to work on non-Windows where os.altsep is None

PyAV-Org/PyAV,mcpv/PyAV,mikeboers/PyAV,xxr3376/PyAV,pupil-labs/PyAV,pupil-labs/PyAV,markreidvfx/PyAV,markreidvfx/PyAV,xxr3376/PyAV,PyAV-Org/PyAV,mcpv/PyAV,markreidvfx/PyAV,pupil-labs/PyAV,mcpv/PyAV,mikeboers/PyAV,xxr3376/PyAV,pupil-labs/PyAV