https://docs.python.org/3/whatsnew/index.html

Keep in mind they have deferred a number of them because of the impending EOL of Python 2.7. There are fewer breaking changes during the latter 3.X series, which should resume in 3.9 or 3.10 now that Python2 has passed on.

Here's a commonly mentioned one:

Changes in Python Behavior: async and await names are now reserved keywords. Code using these names as identifiers will now raise a SyntaxError. (Contributed by Jelle Zijlstra in bpo-30406.)

Note: I think this is a bad idea, I'd rather all these small breaking changes and parser be deferred to 4.X. But they need to be small breaking changes, of course, not a new language.