First bunch of fixes to make debug-toolbar work with Python 3

Hi,
I made these initial fixes to start porting debug-toolbar to Python 3. These are the "easy" ones.
The big problem will be the replace_call decorator from utils.tracking, cause you can no longer get the defining class from method (although you can get its name by __qualname__). So I'd advise changing the signature of replace_call to replace_call(klass, func), where klass will be the defining class(/module) itself and func will be string name of function. I'll do the work, but I want to make sure you'd accept such a change.

Thanks!