tests: ignore "undefined name 'memoryview'" pyflakes error on earlier Python Before this patch, "test-check-pyflakes.t" shows unexpected "undefined name 'memoryview'" error for "mercurial/util.py" on Python 2.6.x or earlier, because they don't define symbol 'memoryview'. This patch introduces excluding patterns into "filterpyflakes.py" to ignore "undefined name 'memoryview'" pyflakes error on Python 2.6.x or earlier