Make setup.py usable without Cython and setuptools

Use the shipped _discid.c if Cython is not available. If setuptools is not
available, don't pass test* and setup_requires arguments to setup. Also drop
use_2to3 since it's not necessary.

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Test that read really raises NotImplemented

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Make 2to3 unnecessary

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
Add notes about pre-built C source files

Signed-off-by: Sebastian Ramacher <sebastian+dev@ramacher.at>
s/"/'/ in some places

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