[docs] [issue28088] Document Transport.set_protocol and get_protocol

[Berker Peksag]

Berker Peksag added the comment:

I just removed self from method signatures (we don't usually include it) and updated the versionadded directives to 3.5.3 since it was also backported to 3.5 in f12a59311885. Please let me know if got the 3.5 version wrong, thanks!

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nosy: +berker.peksag, larry
versions: +Python 3.5, Python 3.7

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