Fix #11583. Changed os.path.isdir to use GetFileAttributes instead of os.stat.
By changing to the Windows GetFileAttributes API in nt._isdir we can figure out if the path is a directory without opening the file via os.stat. This has the minor benefit of speeding up os.path.isdir by at least 2x for regular files and 10-15x improvements were seen on symbolic links (which opened the file multiple times during os.stat). Since os.path.isdir is used in several places on interpreter startup, we get a minor speedup in startup time.