repl.it is a web-based development environment that supports Python along with many other languages. The IDE is pretty slick and you don't even have to sign up to give it a try: Python3 environment on repl.it. This is probably the easiest place to get started with Python3.
pythonAnywhere is another web-based development environment where you can code, compile, and run python programs. It will even host your web apps. The beginner plan is completely free. Paid plans support Jupyter Notebooks.