Add deprecation warning in libdiscid.discid

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Add test cases for read without arguments

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Add cython to setup requires only if it is available

Only use it to ensure that Cython is new enough if we have it available.

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Document known support distributions

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>