plugin_version decorator doesn't work properly on Python 3.6

[xen0l]

A user reported a problem that aws_gate is failing with capture_output=True in plugin_version on Python 3.6. That's possible because capture_output=True was added in Python 3.7.

We should fix this and come up with a better test.

2020-03-20 10:46:33,106 - aws_gate.cli                 - DEBUG - Using AWS profile "saml2aws" in region "eu-west-1"
2020-03-20 10:46:33,106 - aws_gate.utils               - DEBUG - Deferring signal: SIGHUP
2020-03-20 10:46:33,106 - aws_gate.utils               - DEBUG - Deferring signal: SIGINT
2020-03-20 10:46:33,106 - aws_gate.utils               - DEBUG - Deferring signal: SIGTERM
2020-03-20 10:46:33,107 - aws_gate.utils               - DEBUG - Executing "session-manager-plugin --version"
2020-03-20 10:46:33,107 - aws_gate.utils               - DEBUG - Restoring signal: SIGHUP
2020-03-20 10:46:33,107 - aws_gate.utils               - DEBUG - Restoring signal: SIGINT
2020-03-20 10:46:33,107 - aws_gate.utils               - DEBUG - Restoring signal: SIGTERM
Traceback (most recent call last):
  File "/home/david/.local/bin/aws-gate", line 11, in <module>
    main()
  File "/home/david/.local/bin/aws-gate", line 7, in main
    aws_gate.cli.main()
  File "/home/david/.local/lib/python3.6/site-packages/aws_gate/cli.py", line 262, in main
    key_size=args.key_size,
  File "/home/david/.local/lib/python3.6/site-packages/aws_gate/decorators.py", line 30, in plugin_required
    return wrapped_function(*args, **kwargs)
  File "/home/david/.local/lib/python3.6/site-packages/aws_gate/decorators.py", line 38, in wrapper
    version = execute_plugin(["--version"], capture_output=True)
  File "/home/david/.local/lib/python3.6/site-packages/aws_gate/utils.py", line 143, in execute_plugin
    return execute(PLUGIN_NAME, args, **kwargs)
  File "/home/david/.local/lib/python3.6/site-packages/aws_gate/utils.py", line 125, in execute
    result = subprocess.run([cmd] + args, env={"PATH": env}, check=True, **kwargs)
  File "/usr/lib/python3.6/subprocess.py", line 423, in run
    with Popen(*popenargs, **kwargs) as process:
TypeError: __init__() got an unexpected keyword argument 'capture_output'

[larryon]

I can confirm I have the same issue on python 3.6

[xen0l]

@larryon I created a fix for this, but I don't have Python 3.6 at my disposal right now. I will have more time to check it on during the weekend. If you need this to be merged sooner, by any chance you check if this works fine in your setup?

[larryon]

Works fine, THanks @xen0l

[xen0l]

Merged in #352