OrderedDict typing breaks compatibility with Python 3.6

[guhur]

The typing of OrderedDict is introduced in Python 3.7.2 and is not back-ported to 3.6.
I tested with latest versions of typing ( typing_extensions. I am using Python 3.6.12.

A recent update from July is introducing a typing checking of OrderedDict:
https://github.com/allenai/allennlp/blob/ca656fc6bac7be66e566e8f1ba3131f3aa3d7729/allennlp/nn/parallel/ddp_accelerator.py

As a consequence, allennlp is not compatible anymore with 3.6, which is contradictory with your README.

At first sights, it seems to affect allennlp==2.7.0, but not 2.6.0.

Previously, the typing of OrderedDict was managed with quotes: https://github.com/allenai/allennlp/blob/f877fdc30d18178b88c335fbd92722fb77c42d93/allennlp/modules/vision/grid_embedder.py

[epwalsh]

Thanks for bringing this up @guhur!

Even though it seems like it would be an easy fix to maintain support for 3.6, we're only committed to officially supporting the 2 latest stable versions of Python, so I think we will just update the setup.py to require Python 3.7 or higher... unless there is a compelling reason to keep support for 3.6.