Fix pylint and flake8 warnings.

* Mark TabError as noqa to silence linters on Python 3
* Disable mixed_indentation warning for pylint

mixed_tabs_and_spaces.py

@@ -1,7 +1,7 @@
 def square(x):
     sum_so_far = 0
-    for counter in range(x):
-        sum_so_far = sum_so_far + x
-	return sum_so_far
+    for _ in range(x):
+        sum_so_far += x
+	return sum_so_far  # noqa: E999 # pylint: disable=mixed-indentation Python 3 will raise a TabError here
 
 print(square(10))