lint: Disable a buggy `pep8` rule on line breaks and operators.

I'd much rather see something like

    if (thing_is_permissible(user, thing)
            or (user_possesses_hammer(user)
                and glass_break_requested(thing))):

than

    if (thing_is_permissible(user, thing) or
            (user_possesses_hammer(user) and
                glass_break_requested(thing))):

because the former makes the overall logic much easier to scan.
Similarly for a formula full of arithmetic rather than Boolean
operators.  And the actual PEP 8 agrees (though until 2016 it
unfortunately had the opposite advice.)

The upstream linter still applies the backward rule, so disable that.

tools/linter_lib/pep8.py

@@ -79,6 +79,18 @@ def check_pep8(files):
         # "do not assign a lambda expression, use a def"
         # Fixing these would probably reduce readability in most cases.
         'E731',
+
+        # "line break before binary operator"
+        # This is a bug in the `pep8`/`pycodestyle` tool -- it's completely backward.
+        # See https://github.com/PyCQA/pycodestyle/issues/498 .
+        'W503',
+
+        # This number will probably be used for the corrected, inverse version of
+        # W503 when that's added: https://github.com/PyCQA/pycodestyle/pull/502
+        # Once that fix lands and we update to a version of pycodestyle that has it,
+        # we'll want the rule; but we might have to briefly ignore it while we fix
+        # existing code.
+        # 'W504',
     ]
 
     if len(files) == 0: