In your shoes, I wouldn’t fight your framework, just like, as a general principle, I don’t fight City Hall;-). I happen to share your preference for lowercase-with-underscore function names as PEP 8 specifies, but when I’m programming in a framework that forces a different capitalization style, I resign myself to adopting that style too, since I can’t convince the framework to adopt the “better” style, and style inconsistencies (haphazard mixes of different styles) are really worse.
Of course, some mixage is inevitable if you’re using more than one framework… e.g., PyQt with its camelcase, and standard Python library functions with their lowercase and underscores!-). But since frameworks like Qt are often intended to be extended by subclassing, while the standard Python library has less aspects of such a design, in most case where the capitalization style is forced (because you need to override a method, so you can’t choose a different capitalization), it will be forced to camelcase (by Qt), only rarely to lowercase (by the standard Python library). So, I think that adopting Qt style in this case is still the lesser evil.