Fix test_checkstyle.py interpreter constraint

[Eric-Arellano]

Problem

Right now, test_checkstyle.py relies on the assumption that we only allow CPython>=2.7,3.

The tests stop behaving as intended when allowing CPython>=3.6<4, as we will be doing in #6959. This is because the tests are testing how we handle a whitelist of CPython>=3.6, but when we allow this by default, the whitelist no longer matters because the linter will already be working with that interpreter.

Solution

Constrain the test targets to CPython>=3.4,<=3.5, as we no longer support these Python 3 versions, yet they are still meaningful versions. When the whitelist is turned on, the tests will run the linter using either Python 3.4 or 3.5.

Note that constraining to CPython>=3.8 does not work, as Python 3.8 is not yet available so the tests white listing that interpreter will fail due to not finding a corresponding interpreter.

Other solution attempted

I first tried to constrain PythonSetup.global_instance().interpreter_constraints to only allow Python 2.7, as we do now.

I got the constraint to work, but the tests were still failing. I found that this is due to the function PythonSetup.compatibility_or_constraints, which no matter what uses the passed compatibility constraints when given, rather than first trying to read the target's compatibility. So, the Python 3 targets were improperly resolving as Python 2 compatible.

[Eric-Arellano]

Let me know if I should be adding @skip_unless_python34_or_python35. I didn't do so already because we don't have this function (nor functionality to allow either interpreter), and it adds a lot of code to have a decorator on every function in test_checkstyle.py (unless there's a way to add it to every test function?).

Thank you!

[Eric-Arellano]

CI is failing because PythonInterpreterCache cannot find a 3.4 or 3.5 interpreter:

"No valid interpreters found for targets: [PythonLibrary(a/python:fail3)]\n(filters: (u'CPython>=3.4,<=3.5',))"

I can't figure out why this is and how to fix it. Ubuntu Xenial comes by default with Python 3.5, and we were using this in CI for a week, so it's definitely there and discoverable. Looking at the PythonInterpreterCache, I'm thinking it's looking at what's already in the cache and failing to search for additional interpreter:

# this is from PythonInterpreterCache.setup()`
interpreters.extend(self._setup_cached(filters=filters))
      if not interpreters or unsatisfied_filters():
        interpreters.extend(self._setup_paths(setup_paths, filters=filters))

[jsirois]

Try <3.6 since 3.5.1 is not <=3.5 (for example).