driver/externalconsoledriver: use bufsize=0 for opening external process

The ExternalConsoleDriver works on top of a command executed on the local
computer. Until now the system default of io.DEFAULT_BUFFER_SIZE
(typically 8192 bytes) was used for bufsize. This does not work well
for this use case, we want unbuffered reads and writes. So set the
bufsize to 0, see
https://docs.python.org/3/library/subprocess.html#popen-constructor .

Signed-off-by: Bastian Krause <bst@pengutronix.de>

Author: Bastian-Krause

labgrid/driver/externalconsoledriver.py

@@ -33,7 +33,7 @@ def open(self):
             return
         cmd = shlex.split(self.cmd)
         self._child = subprocess.Popen(
-            cmd, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=None
+            cmd, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=None, bufsize=0
         )
 
         # make stdout non-blocking