Fix bytes issue in wrapper.py

Bug: 90
Change-Id: I70cf5a0d1d092f0f16eff68dc59a26599488bbea
diff --git a/pw_env_setup/py/pw_env_setup/cipd_setup/wrapper.py b/pw_env_setup/py/pw_env_setup/cipd_setup/wrapper.py
index ef9aa58..7a23255 100755
--- a/pw_env_setup/py/pw_env_setup/cipd_setup/wrapper.py
+++ b/pw_env_setup/py/pw_env_setup/cipd_setup/wrapper.py
@@ -50,7 +50,9 @@
     try:
         with open(os.devnull, 'w') as outs:
             PW_ROOT = subprocess.check_output(
-                ['git', 'rev-parse', '--show-toplevel'], stderr=outs).strip()
+                ['git', 'rev-parse', '--show-toplevel'],
+                stderr=outs,
+            ).strip().decode('utf-8')
     except subprocess.CalledProcessError:
         PW_ROOT = None