What would python add that Javascript doesn't already let you do? Granted it can make programming easier in my opinion, but think of the complexity it would add. Any major website would need to either have two versions of their code (one JS, one Python), or more likely just use Javascript (you can't turn away visitors because their browser doesn't support your favorite language)
That doesn't even begin to handle the backwards compatibly argument. We still have people using browsers like IE6, so for a long period of time you would need to write code in both languages.
This is the wrong approach. Javascript is rapidly becoming the "bytecode" of browsers. Look at established projects like Haxe and GWT which take higher level languages and compile them down to low-level javascript. Pyjamas is even a python port of Gwt, not sure how well tested.
This approach solves many problems. Browser developers only have to support one language (javascript), and already projects like the V8 engine are making that fast and standardized. Developers can use the language of their choice.
It's what Java should have been in the first place. It's a pity that this is all built on an oddball language like javascript, but perhaps that's the only way this could have happened because there's no legacy javascript outside the browser.
I'm no javascript guru, but let's please keep the number of languages embedded in browsers limited to the absolute minimum.
And let's get rid of flash while we're 'discussing' this.