mb: decode process output for Python3

CL from chromium's mb:
https://chromium-review.googlesource.com/c/chromium/src/+/3321055

Bug: webrtc:13839
Change-Id: Ie90eceaff8c824aa23bc27149dc88b9ac4cd1936
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/256006
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Commit-Queue: Jeremy Leconte <jleconte@google.com>
Cr-Commit-Position: refs/heads/main@{#36244}

tools_webrtc/mb/mb.py

@@ -1213,6 +1213,8 @@ class MetaBuildWrapper:
                            stderr=subprocess.PIPE,
                            env=env)
       out, err = p.communicate()
+      out = out.decode('utf-8')
+      err = err.decode('utf-8')
     else:
       p = subprocess.Popen(cmd, shell=False, cwd=self.src_dir, env=env)
       p.wait()