Python in the browser would ultimately lead to all the same problems as Javascript in the browser. In a few years, we'll be hearing: Clojure in the browser, Scala in the browser, Haskell in the browser.
I've been programming in Python full-time for the past 6 months, and it's main deficiency are the same as JavaScript's: no (optional) static typing - I'm really sick of writing checks whether my function got values of the correct types, and unittests testing if a class's/function's signature changed in an unexpected way.
What we need, in my opinion, is a dynamic language with powerful optional static typing (unlike Dart, where types are just comments), with sane object-orientedness, and support for immutable values. The core has to be really simple, but the language has to be powerful enough so that libraries can provide the missing functionality (math - rationals, matrices, ...; concurrency - channels, isolates, ...; GUI, IO (with formats, ...)).