Add the possibility to follow symlinks on copytree also for py3

Hello,

It's an ansqer of https://github.com/pypa/pip/pull/1311/files#r7640350

If a project contains true NTFS symlinks it require elevated privileges indeed (source: [@docs.python: os.symlink](http://docs.python.org/3.3/library/os.html?highlight=os.symlink#os.symlink) and [@wikipedia: NTFS_symbolic_link](https://en.wikipedia.org/wiki/NTFS_symbolic_link))

- [x] Fix shutil.copytree for py3 when a symlink points to a directory
- [x] Prevent circular symbolic links
- [x] Add test with circular symbolic links
- [ ] Turn off symlinks copy on Path::copytree method