Decode output from subprocess before performing string operations

bootstrap/bootstrap.py

@@ -29,7 +29,7 @@ def get_os_release_variable(key):
     return subprocess.check_output([
         '/bin/bash', '-c',
         "source /etc/os-release && echo ${{{key}}}".format(key=key)
-    ]).trim()
+    ]).decode().trim()
 
 def main():