Here is a link to cpython 2.7 libraries: http://hg.python.org/cpython/file/70274d53c1dd/Lib
Those that are implemented in C are in the "Modules" folder from the top level.
I think you [OP] will get the most benefit from reading libraries that you use often because it will give you some direction.
If there are 3rd party libraries you use heavily those will be interesting too.
It is very very difficult to just pick up megabytes of code and start reading them and find it useful. You will be able to pick up style and conventions but not really high-level engineering decisions.
My suggestion is to take software that you use regularly and run it through a debugger. Since you use it you know the problem domain. And with the debugger the code you're reading gets real context. I've done this with Git for example.
And also look at the design documents for big OSS projects.