Raise EndOfStream from None in MemoryObjectReceiveStream.receive()

[agronholm]

Changes

Changes EndOfStream to be raised from None to avoid AttributeError: 'MemoryObjectItemReceiver' object has no attribute 'item' to be included in the traceback, leading many to think this is a bug when in fact it was only an implementation detail.

Checklist

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• You've added tests (in tests/) added which would fail without your patch
• You've updated the documentation (in docs/, in case of behavior changes or new
  features)
• You've added a new changelog entry (in docs/versionhistory.rst).

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Updating the changelog

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If, say, your patch fixes issue #123, the entry should look like this:

- Fix big bad boo-boo in task groups
  (`#123 <https://github.com/agronholm/anyio/issues/123>`_; PR by @yourgithubaccount)

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