Fix more lint warnings reported by flake8

Fixes:
test_isort.py:33:1: F403 'from isort.pie_slice import *' used; unable to detect undefined names
test_isort.py:33:1: F401 'isort.pie_slice.*' imported but unused
test_isort.py:366:1: E302 expected 2 blank lines, found 1
test_isort.py:891:51: E261 at least two spaces before inline comment
test_isort.py:1340:1: E303 too many blank lines (3)
test_isort.py:1792:1: E302 expected 2 blank lines, found 1
test_isort.py:2038:5: E303 too many blank lines (2)
isort/isort.py:165:114: E502 the backslash is redundant between brackets
isort/isort.py:413:13: E303 too many blank lines (2)
isort/isort.py:563:24: E713 test for membership should be 'not in'
isort/isort.py:726:16: E713 test for membership should be 'not in'
isort/isort.py:831:72: E713 test for membership should be 'not in'
isort/isort.py:838:72: E713 test for membership should be 'not in'
isort/isort.py:885:115: E713 test for membership should be 'not in'
isort/isort.py:909:36: E713 test for membership should be 'not in'
isort/pie_slice.py:26:1: F401 'functools' imported but unused
isort/pie_slice.py:28:1: F401 'numbers.Integral' imported but unused
isort/pie_slice.py:156:5: F401 'codecs' imported but unused
isort/settings.py:39:29: E261 at least two spaces before inline comment