Refactor check_banned_imports.py to allow Python 3 files to import subprocess

[Eric-Arellano]

Problem

Once we drop Python 2, we will start directly importing the stdlib module for subprocess and will stop using subprocess32 and pants.util.process_handler.subprocess. However, our check_banned_imports.py would not allow this to happen.

At the same time, we would like to keep checking that our Python 2 files are valid, so we don't want to remove the check outright.

Solution

Extract out a filter_files() function that will remove any files without the given snippet. This allows us to identify and save in memory all of our Python 2 files, then to call check_banned_import on only those files.

We check for the presence of from __future__ imports, which are only used in our codebase for Python 2 compatibility.

Result

We can now import subprocess in Python 3-only files.

There is a 1-second slowdown in the performance of that script. After we finish dropping Python 2, we could reclaim this second by removing both the filter to get Python 2 only files and removing the subprocess check entirely.

[blorente]

Couple of nits, but looks great!