Use pylint 2.7 for depot_tools

This includes a few fixes for specific errors, and disables several new
warnings introduced in this version, in order to allow for an incremental migration.

Bug:1262286
Change-Id: I4b8f8fc521386419a3121bbb07edc8ac83170a94
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3413679
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Aravind Vasudevan <aravindvasudev@google.com>

testing_support/fake_repos.py

@@ -49,7 +49,7 @@ def read_tree(tree_root):
       dirs.remove(d)
     for f in [join(root, f) for f in files if not f.startswith('.')]:
       filepath = f[len(tree_root) + 1:].replace(os.sep, '/')
-      assert len(filepath), f
+      assert len(filepath) > 0, f
       with io.open(join(root, f), encoding='utf-8') as f:
         tree[filepath] = f.read()
   return tree