From https://en.wikipedia.org/wiki/Foreign_function_interface
the ctypes module can load C functions from shared libraries/DLLs on-the-fly and translate
You're confusing lower case cdll (which is a LibraryLoader) with upper case CDLL, which is the constructor for shared libraries.
cdll
This code will work as expected:
libc = ctypes.CDLL("/lib/x86_64-linux-gnu/libc.so.6")