Drop support for cdecimal

[dbrgn]

From https://stackoverflow.com/a/8192918/284318:

As of Python 3.3, the cdecimal implementation is now the built-in
implementation of the decimal standard library module, so you don't
need to install anything. Just use decimal.

Since combining decimal and cdecimal can result in weird bugs, and
since I will drop Python 2 support in the next release anyways,
supporting cdecimal doesn't make sense anymore.

[coveralls]

Coverage decreased (-1.3%) to 61.111% when pulling f3790e9 on drop-cdecimal into 7f22f98 on master.