[infra] Decode process output for Python3

This ports:
https://crrev.com/c/3321055

No-Tree-Checks: true
Bug: chromium:1292013
Change-Id: I2ab939c43744725885329330f52dce1ef0a48ab7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3466795
Auto-Submit: Michael Achenbach <machenbach@chromium.org>
Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79111}

tools/mb/mb.py

@@ -1150,6 +1150,8 @@ class MetaBuildWrapper(object):
                            stdout=subprocess.PIPE, 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.chromium_src_dir,
                            env=env)