Python package should not require `readline`

[laserson]

I believe this is often provided by default, and can cause lots of issues when the user is forced to build it. Could we remove this bit of code?

https://github.com/cloudera/cm_api/blob/master/python/setup.py#L31-L34

[asmeurer]

Or at least check if it is already installed with import readline. The problem is that distributions that come with readline have it packaged in the standard library, meaning it will not be seen as installed by setuptools.