tests: make filterpyflakes.py read target files relatively to cwd Before this patch, 'filterpyflakes.py' reads target files relatively to own location. But this prevents third party tools from using it in own source tree, because their files are placed separately from 'filterpyflakes.py'. In fact, 'test-check-pyflakes.t', which is the only user of 'filterpyflakes.py', changes current working directory (cwd) to the root of "test target" source tree before using it. Therefore, composing the root of source tree in 'filterpyflakes.py' is redundant. This patch makes 'filterpyflakes.py' read target files relatively to cwd by invoking 'open()' without any path composition. This also removes importing 'os' module, because there is no user of it after this patch. This is a one of preparation of issue4677.