compatibility astropy units non standard

in tools/fits.py, this

    try:
        dsthdr['BUNIT'] = (unit.to_string('fits'), 'data unit type')
    except u.format.fits.UnitScaleError:
        dsthdr['BUNIT'] = (fix_unit_write(str(unit)), 'data unit type')

should be

    try:
        dsthdr['BUNIT'] = (unit.to_string('fits'), 'data unit type')
    except:
        dsthdr['BUNIT'] = (fix_unit_write(str(unit)), 'data unit type')