debugcommands: move away from line buffered output on binary stream Line buffering on binary file objects is apparently undefined behavior in Python and emits a RuntimeWarning on Python 3.8. See https://bugs.python.org/issue32236. This commit changes the I/O logging file descriptor from line buffered to unbuffered to work around this. I'm no fan of unbuffered I/O for performance reasons. But I don't think it is an issue here given the nature of the code. With this change, test-ssh-proto.t now passes on Python 3.8. Differential Revision: https://phab.mercurial-scm.org/D7948