Tornado 6.3

[bdarnell]

The next version of Tornado will be 6.3. Changes I'd like to get in include

• Undeprecate most of the things from 6.2 (issue: Reverse some deprecation warnings from Tornado 6.2 #3216, PR: Revert some deprecations, following asyncio changes #3223)
• Dropping support for Python 3.7. This is less necessary since we're no longer deprecating AsyncTestCase, but 3.7 just has a few more months of support left so it's not worth undoing that change. (and it's necessary to use samesite cookies)
• Greatly improved performance for large websocket messages sent in multiple fragments (no longer quadratic)
• Deprecate all the xsrf_token stuff in favor of using samesite on auth cookies. PR: web: Deprecate xsrf_cookies #3226
• Figure out how to support cookie prefixes (Feature request: Cookie prefixes #2968) Needs more design work so won't be making it in to 6.3.
• Multi-threaded WSGIContainer (Add executor support to WSGIContainer #3210)
• XSRF cookies v3 I didn't build a full "v3" of xsrf_cookies but we now have the option to rename the cookie to use a host prefix.