If you're looking for a full-featured IDE, I don't think you need to look any further than PyCharm, however, Sublime with it's PEP 8 Linter, and CodeIntel plugins will give you a lighter-weight yet effective solution.

You can even run your tests in Sublime's console if that's something you'd want to do, or use the terminal (as I ended up doing).

Similarly, for vim as people have pointed out, if that's something you're more comfortable with.