virtualenv_wrapper: fix string handling

We use the output of this command in os.path.join, so make sure it's
a string and not bytes.  This fixes a bunch of unittests too.

BUG=chromium:997354
TEST=`./run_tests` passes

Change-Id: I1f371b76d710c393b302fb3c18d2916f4c7e7fe2
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1869344
Reviewed-by: Jack Rosenthal <jrosenth@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
diff --git a/scripts/virtualenv_wrapper.py b/scripts/virtualenv_wrapper.py
index 3985cf2..3b710c9 100755
--- a/scripts/virtualenv_wrapper.py
+++ b/scripts/virtualenv_wrapper.py
@@ -38,7 +38,7 @@
   return subprocess.check_output([
       _CREATE_VENV_PATH,
       _REQUIREMENTS,
-  ]).rstrip()
+  ]).rstrip().decode('utf-8')
 
 
 def _ExecInVenv(venvdir, args):