bpo-33671 / shutil.copyfile: use memoryview() with dynamic size on Windows (#7681)

bpo-33671
* use memoryview() with size == file size on Windows, see https://github.com/python/cpython/pull/7160#discussion_r195405230
* release intermediate (sliced) memoryview immediately
* replace "OSX" occurrences with "macOS"
* add some unittests for copyfileobj()