Issue #20951: SSLSocket.send() now raises either SSLWantReadError or SSLWantWriteError on a non-blocking socket if the operation would block. Previously, it would return 0.

Patch by Nikolaus Rath.
Add porting note for issue #20951.
Issue #21057: TextIOWrapper now allows the underlying binary stream's read() or read1() method to return an arbitrary bytes-like object (such as a memoryview).

Patch by Nikolaus Rath.
Fix failure in test_poplib after issue #20951.
Issue #21321: itertools.islice() now releases the reference to the source iterator when the slice is exhausted.

Patch by Anton Afanasyev.
Issue #21321: itertools.islice() now releases the reference to the source iterator when the slice is exhausted.

Patch by Anton Afanasyev.