> I recall a "write1" function which was well defined: limited to 1 syscall, don't try (or maybe only on the very specific case of EINTR). But I'm not sure that it still exists in the io module of Python 3.

Oops, in fact it is read1:
https://docs.python.org/dev/library/io.html#io.BufferedIOBase.read1
"Read and return up to size bytes, with at most one call to the underlying raw stream’s read()"