deprecation warning for DEFAULT_DEVICE shows always

I did check that the deprecationWarning works, but I didn't see until the release that this triggers on any `import discid` because python-discid is the one using the function that triggers the warning.
That is: #34 doesn't work as expected.

it looks like there is no way to trigger a deprecation warning for constants at all.

So we either include it without warning, possibly having this remain in scripts without problems until I remove it completely
or I just remove DEFAULT_DEVICE.

There might be weird workarounds with returning something special as DEFAULT_DEVICE and detecting this in read and issue the warning then, but that is a mess to maintain and the warning isn't displayed when the constant is used, but later on (or possibly never).