* 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.

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.

Passing a unicode url to get() always worked (assuming the url contained only ASCII), but it didn't work to post(), put(), or delete() if query parameters were included. This change fixes the latter cases.

On Python 3, cgi.FieldStorage has a __del__ method that closes the underlying file [1]. This means that if the copy made from UnicodeMultiDict._decode_value is garbage collected, the file underlying the original FieldStorage will be closed! Fix this by not copying FieldStorage if it is not required by decode_keys=False. I cannot think of a nice way to fix this problem if decode_keys=True.

[1] https://github.com/python/cpython/commit/f79126f373a9d5c9b584a8db736fe490fcbfa77a

As the test shows, this fixes passing boths params and upload_files on Python 3.

Replace `isinstance(self.content, file)` with a duck type for the `read()` method. Having a read method is what PEP 333 defines as the minimum requirement for a "file-like" object.