bpo-1539381: Implement SpooledTemporaryFile.readinto

[alanjds]

SpooledTemporaryFile have no readinto because of StringIO absence of it.

However, SpooledTemporaryFile have 3 internal "modes": BytesIO, StringIO and TemporaryFile.

This PR allows it use and expose .readinto of the modes that allows it. And the one that does not will raise its exception.

https://bugs.python.org/issue1539381

[the-knights-who-say-ni]

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[corona10]

@alanjds
Brett Cannon already rejected this issue.
I think you should send a new discussion into Python-dev.

[alanjds]

Ok. Thanks. I will take a look on how to send discussions there.

Should it go to python-ideas before, or is not necessary?

[corona10]

@alanjds
https://mail.python.org/mailman/listinfo/python-dev

[corona10]

I've send simillar discussion before but I only send dev.

[iritkatriel]

Closing as https://bugs.python.org/issue1539381 was rejected.