Importing chainer fails on Python 3.5.1

[okapies]

This is a bug reported by @hvy. When importing the current master version of chainer (070deb5) on Python 3.5.1, it fails with RuntimeError because it uses the older builtin typing module which doesn't support @overload annotation outside a stub file.

$ python
Python 3.5.1 (default, Nov 15 2018, 17:50:51) 
[GCC 5.5.0 20171010] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import chainer
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/user/chainer/chainer/__init__.py", line 18, in <module>
    from chainer import links  # NOQA
  File "/home/user/chainer/chainer/links/__init__.py", line 3, in <module>
    from chainer.links.activation.maxout import Maxout  # NOQA
  File "/home/user/chainer/chainer/links/activation/maxout.py", line 6, in <module>
    from chainer import link
  File "/home/user/chainer/chainer/link.py", line 1144, in <module>
    class ChainList(Link, collections_abc.MutableSequence):
  File "/home/user/chainer/chainer/link.py", line 1182, in ChainList
    @tp.overload  # NOQA
  File "/home/user/.pyenv/versions/3.5.1/lib/python3.5/typing.py", line 1184, in overload
    raise RuntimeError("Overloading is only supported in library stubs")
RuntimeError: Overloading is only supported in library stubs