Do not allow python2 to be added to salt-ssh tar by default

[Ch3LL]

What does this PR do?

After dropping python 2 support from Salt we should not be adding python 2 modules to the salt-ssh binary by default. This PR addresses this by removing the code that automatically looked for python 2 modules and added them to the tar. This also ensures that ssh_ext_alternatives can still work with older python 2 supported salt versions. For example, when using ssh_ext_alternatives you can use the 2019.2.3 version of salt that is copied over in the tar. This version still supports Python 2 so we should still allow a user to use this version when using the ssh_ext_alternatives feature.

What issues does this PR fix or reference?

Fixes: #57647

Previous Behavior

Misleading error when python 3 is not installed on the target host:

        ERROR: Python version error. Recommendation(s) follow:
        - Install Python 2.6/2.7 Salt dependencies on the Salt SSH 
          master to interact with Python 2.6/2.7 targets
        - Install Python 3 on the target machine(s)

and python 2 modules added to the tar even though they will not be used.

New Behavior

New Error to inform user they need to install Python 3 on target host.

        ERROR: Python version error. Recommendation(s) follow:
        - Install Python 3 on the target machine(s)
        - You can use ssh_pre_flight or raw shell (-r) to install Python 3

Merge requirements satisfied?

• Docs
• Changelog - https://docs.saltstack.com/en/master/topics/development/changelog.html
• Tests written/updated

Commits signed with GPG?

Yes

[Ch3LL]

re-run full all

[Ch3LL]

re-run full all

[sagetherage]

re-run full all

[Ch3LL]

re-run full all

[Ch3LL]

re-run full all

[DmitryKuzmenko]

LGTM

[s0undt3ch]

Suggested change

	@pytest.fixture()
	@pytest.fixture

[s0undt3ch]

Suggested change

	"extension_modules": tmpdir.join("extmods"),
	"extension_modules": tmpdir.join("extmods").strpath,

[Ch3LL]

@s0undt3ch ready for re-review

[s0undt3ch]

Do we still need this if _six.PY2 check?

[Ch3LL]

Yes. This keeps support for https://docs.saltstack.com/en/master/topics/ssh/ssh_ext_alternatives.html which allows you to bundle up an old version of salt that uses python 2 to use on the target host. This PR removes salt-ssh trying to automatically add python2 libraries automatically, but if a user is intentionally using ssh_ext_alternatives I think they should still be able to add python 2 libraries for older versions of salt.

[s0undt3ch]

Are we sure on 3.0, salt is only meant to work with 3.5, right?
Or for this piece of code it doesn't really matter?

[Ch3LL]

I will re-word this. You can even actually use 2.7 if using an older version of salt.