* Use is_alive instead of isAlive for Python 3.9 compatibility.

* Use encodebytes instead of deprecated encodestring.

* Fix Python 2 and 3 compatibility for base64.

* Avoid some reference cycles through tracebacks in httpserver.py

* Handle io.UnsupportedOperation from socket.tell()

On Python 3, socket.makefile() returns an object with a tell() method, but one that always raises io.UnsupportedOperation.

If we don't want to parse the request body with cgi, we can just not use cgi rather than building a fake request.

* Ensure unicode URLs work in TestApp.
* Make LimitedLengthFile file return empty bytes.
* Protect against accidental close in FieldStorage.