google.com web pages with ".py" extension?
Most likely you request a file that's ending in .py that spits out the usual HTML et. al. The file is executed on the server side, not in your browser.
But again, that's a URL and it could point to any resource. Could be anything. Like a lot of websites use pretty URLs to point you to something , except in this case it's not pretty. (Behind the scenes there are routers, rewrite rules sometimes, etc. to do this.)
Take, for instance, support.google.com/mail/bin/answer.py?hl=en&answer=13287. What is that answer.py? How does this work? I'm 99.99% sure that browsers don't have the ability to interpret python code (yet) like javascript / PHP. So what is this? Is it some Python webframework?