usbtmc.py resp.tobytes version check python < 3.2

[Dedier]

In read_raw the python version is checked for tobytes or tostring.
This checkes for pythen 3.0, but tobytes was introduced in 3.2.
`

        if sys.version_info >= (3, 2):
            resp = resp.tobytes()
        else:
            resp = resp.tostring()

`

[alexforencich]

Should be fixed now.