Fix setuptools's Python 2 deprecation warning with Python linters

[Eric-Arellano]

Setuptools 45.x is the last to support Python 2. It has an incredibly noisy stderr message to make sure users are aware of this:

/private/var/folders/sx/pdpbqz4x5cscn9hhfpbsbqvm0000gn/T/process-executionUnBXvi/pex_root/install/setuptools-45.2.0-py2-none-any.whl.91f9e7e571716e18411266133f3db5a0212dfa92/setuptools-45.2.0-py2-none-any.whl/pkg_resources/py2_warn.py:22: UserWarning: Setuptools will stop working on Python 2
************************************************************
You are running Setuptools on Python 2, which is no longer
supported and
>>> SETUPTOOLS WILL STOP WORKING <<<
in a subsequent release (no sooner than 2020-04-20).
Please ensure you are installing
Setuptools using pip 9.x or later or pin to `setuptools<45`
in your environment.
If you have done those things and are still encountering
this message, please comment in
https://github.com/pypa/setuptools/issues/1458
about the steps that led to this unsupported combination.
************************************************************
  sys.version_info < (3,) and warnings.warn(pre + "*" * 60 + msg + "*" * 60)

By tweaking our default versions, we can make sure our users don't get this noise when running ./pants fmt2 and ./pants lint2.

[asherf]

I don't follow.... Why are we running those tools (bandit, flake8) under a python2 interpreter ?
Even if they check/run on a python 2 code base, we don't have to run them in a python 2 interpreter....

[Eric-Arellano]

I don't follow.... Why are we running those tools (bandit, flake8) under a python2 interpreter ?
Even if they check/run on a python 2 code base, we don't have to run them in a python 2 interpreter....

For Bandit and Flake8, Pants figures out the interpreter constraints first based off of the specific target's compatibility field, then --python-setup-interpreter-constraints. It's possible to run those tools on Python 2 targets, and we need to keep that support so long as those tools keep Python 2 support.

For isort, we tell Pex to use whatever the default_interpreter_constraints are, which are hardcoded by Pants but can also be configured via --isort-interpreter-constraints. Originally in this PR I tried setting isort to always use Python 3.6 per #9131 (comment) because it doesn't matter what version we use for isort. But, CI kept hanging so I'm trying out if setting setuptools<45 and still allowing Python 2 fixes things. In isort 5.0, it will drop support for Python 2 so we'll able to remove this workaround.

[Eric-Arellano]

Weird. Confirmed that removing the interpreter constraints from isort and setting setuptools<45 does fix the hang in CI. I have no idea why that is, but we'll handle the issue once we upgrade to isort 5.0 I suppose.