typing is part of standard library from python3.6 (gives errors on python3.7)

[MartinoMensio]

installing it on python3.7 causes AttributeError: type object 'Callable' has no attribute '_abc_registry'
refers #1457 and python/typing/issues/573.

To reproduce the issue simply install allennlp in a new environment with python3.7 and then run the command allennlp predict https://s3-us-west-2.amazonaws.com/allennlp/models/srl-model-2018.05.25.tar.gz srl-examples.jsonl given on the demo site https://allennlp.org/models.
It will produce the following error:

Traceback (most recent call last):
  File "/usr/lib64/python3.7/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/lib64/python3.7/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/home/martino/kmi/allennlp_playground/venv/lib/python3.7/site-packages/allennlp/run.py", line 15, in <module>
    from allennlp.commands import main  # pylint: disable=wrong-import-position
  File "/home/martino/kmi/allennlp_playground/venv/lib/python3.7/site-packages/allennlp/commands/__init__.py", line 1, in <module>
    from typing import Dict
  File "/home/martino/kmi/allennlp_playground/venv/lib/python3.7/site-packages/typing.py", line 1347, in <module>
    class Callable(extra=collections_abc.Callable, metaclass=CallableMeta):
  File "/home/martino/kmi/allennlp_playground/venv/lib/python3.7/site-packages/typing.py", line 1003, in __new__
    self._abc_registry = extra._abc_registry
AttributeError: type object 'Callable' has no attribute '_abc_registry'

The solution is not to install the backport package typing on the latest python versions.

[joelgrus]

good catch. AllenNLP requires at least Python 3.6 to run in the first place, so can you modify the PR to just remove typing altogether? thanks