fix linter

Author: jiasli

src/azure-cli/azure/cli/__main__.py

@@ -103,7 +103,6 @@ def cli_main(cli, args):
                         else:
                             upgrade_exit_code = subprocess.call(cmd, shell=platform.system() == 'Windows')
                     else:
-                        import os
                         devnull = open(os.devnull, 'w')
                         cmd.append('-y')
                         upgrade_exit_code = subprocess.call(cmd, shell=platform.system() == 'Windows', stdout=devnull)