Add missing contextvars dependency in salt.version

[vzhestkov]

What does this PR do?

PR: https://github.com/openSUSE/salt/pull/650 is causing test fail due to missing contextvar dependency.

This change intended to fix the following error failing the test tests/unit/states/test_pip_state.py::PipStateInstallationErrorTest::test_importable_installation_error:

E           b'Traceback (most recent call last):\n  File "/__w/salt/salt/salt/utils/ctx.py", line 5, in <module>\n    import _contextvars as contextvars\nModuleNotFoundError: No module named \'_contextvars\'\n\nDuring handling of the above exception, another exception occurred:\n\nTraceback (most recent call last):\n  File "<string>", line 4, in <module>\n  File "/__w/salt/salt/salt/__init__.py", line 175, in <module>\n    import salt._logging  # isort:skip\n  File "/__w/salt/salt/salt/_logging/__init__.py", line 12, in <module>\n    from salt._logging.impl import (\n  File "/__w/salt/salt/salt/_logging/impl.py", line 29, in <module>\n    import salt.utils.ctx\n  File "/__w/salt/salt/salt/utils/ctx.py", line 8, in <module>\n    import contextvars\nModuleNotFoundError: No module named \'contextvars\'\n'

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• [ ] Docs
• [ ] Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• [ ] Tests written/updated

Commits signed with GPG?

Yes/No

Please review Salt's Contributing Guide for best practices.

See GitHub's page on GPG signing for more information about signing commits with GPG.