tools/pyboard.py: Capture stdout for pts line.

The pts line printed by qemu-system-arm goes to stdout, not stderr.

Redirect stderr to stdout in case other tools do print to stderr.
Signed-off-by: Damien George <damien@micropython.org>

tools/pyboard.py

@@ -235,9 +235,9 @@ class ProcessPtyToTerminal:
             preexec_fn=os.setsid,
             stdin=subprocess.PIPE,
             stdout=subprocess.PIPE,
-            stderr=subprocess.PIPE,
+            stderr=subprocess.STDOUT,
         )
-        pty_line = self.subp.stderr.readline().decode("utf-8")
+        pty_line = self.subp.stdout.readline().decode("utf-8")
         m = re.search(r"/dev/pts/[0-9]+", pty_line)
         if not m:
             print("Error: unable to find PTY device in startup line:", pty_line)