Fail if Python 2 values are specified - #887

PR Checklist

Please check if your PR fulfills the following requirements:

• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature (please, look at the "Scope of the project" section in the README.md file)
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Python 2 usages of the rules will immediately fail. Examples include:

py_binary(..., python_version="PY2")
py_library(..., srcs_version="PY2")
py_runtime_pair(..., py2_runtime="//something")