Bug 1436639 - [lint] Make sure flake8 is run with same python as |mach lint| was, r=jmaher

This fixes a bug which can happen when the default version of python differs from
the version of python used with mach.

For example, mach explicitly looks for python2.7. This means running |mach lint -l flake8|
should also run flake8 with version 2.7. But if the default is python3, and flake8 is also
installed there, the subprocess call that invokes flake8 will run under python3. This can
lead to errors like "undefined name 'basestring'" and other 2to3 gotchas.

This patch ensures that we run:
python2.7 -m flake8

which explicitly runs flake8 against the same interpreter as mach, no matter the default.

MozReview-Commit-ID: HSuMzDsAvsW

tools/lint/python/flake8.py

@@ -6,9 +6,9 @@ import json
 import os
 import signal
 import subprocess
+import sys
 from collections import defaultdict
 
-import which
 from mozprocess import ProcessHandlerMixin
 
 from mozlint import result
@@ -90,23 +90,12 @@ class Flake8Process(ProcessHandlerMixin):
         signal.signal(signal.SIGINT, orig)
 
 
-def get_flake8_binary():
-    """
-    Returns the path of the first flake8 binary available
-    if not found returns None
-    """
-    try:
-        return which.which('flake8')
-    except which.WhichError:
-        return None
-
-
 def _run_pip(*args):
     """
     Helper function that runs pip with subprocess
     """
     try:
-        subprocess.check_output(['pip'] + list(args),
+        subprocess.check_output([sys.executable, '-m', 'pip'] + list(args),
                                 stderr=subprocess.STDOUT)
         return True
     except subprocess.CalledProcessError as e:
@@ -143,9 +132,8 @@ def setup(root):
 
 
 def lint(paths, config, **lintargs):
-    binary = get_flake8_binary()
     cmdargs = [
-        binary,
+        sys.executable, '-m', 'flake8',
         '--format', '{"path":"%(path)s","lineno":%(row)s,'
                     '"column":%(col)s,"rule":"%(code)s","message":"%(text)s"}',
     ]