New patch in attachment. Changes:

- docs

- replaced select() / poll() with the new selectors module

- file position is always updated both on return and on error; this means file.tell() is the designated way to know how many bytes were sent

- replaced sendall() with send() so that we can count the number of bytes transmitted (related and rejected proposal: https://mail.python.org/pipermail/python-ideas/2014-April/027689.html)

- send() now uses memoryview() for better performances to re-transmit data which was not sent by the first send() call

- pre-emptively raise exception if file is not opened in binary mode

- tests for ssl module

I've tried to work on Windows TransmitFile support but I got stuck as I'm not sure how to convert a file object into a HANDLE in C. I suppose Windows support can also be added later as a separate ticket and in the meantime I'd like to push this forward.

Open questions: 

- Is the current return value desirable (do we really care if os.sendfile() was used internally?)? Should the returned tuple also include the number transmitted bytes?

- default blocksize: Charles-François was suggesting to remove the blocksize argument; FWIW I've made some quick benchmarks by using "time" cmdline utility with different blocksizes and I didn't notice substantial difference. I still think a blocksize parameter is necessary in case we fallback on using send() and also for consistency with ftplib's storbinary() method which will be involved later (issue 13564).