Mitogen was leaving the stdout and stderr of subprocesses in non-blocking
mode. When Python code ran in the remote process created by Mitogen calls such
as `print(long_string)` or `os.stout.write(bigger_than_the_buffer)` sometimes
raised `BlockingIOError`, or similar.
This change
- Removes code in `mitogen.core.Side` that set blocking/non-blocking mode
- Adds blocking/non-blocking control to `os.mitogen.pipe()` and a new
function `mitogen.core.socketpair()`
- Replaces `mitogen.core.set_block` and `mitogen.core.set_nonblock`
with `mitogen.core.set_blocking`, mirroring `os.set_blocking`
- Updates call sites as appropriate
- Adds tests for new functions and arguments
- Adds a regression test for subprocess stdio blocking/non-blocking
fixes#712
Alex Willmer