python 3.7 compatability

[shimwell]

running in python 3.7 brings up this error

---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)
<ipython-input-11-86f9c5199754> in <module>
----> 1 from cad_to_h5m import cad_to_h5m
      2 
      3 cad_to_h5m(
      4     files_with_tags=[
      5         {'cad_filename':'plasma.stp', 'material_tag':'mat_plasma'},

/opt/conda/lib/python3.7/site-packages/cad_to_h5m/__init__.py in <module>
----> 1 from .core import cad_to_h5m

/opt/conda/lib/python3.7/site-packages/cad_to_h5m/core.py in <module>
      2 import os
      3 import json
----> 4 from typing import Dict, List, TypedDict, Optional
      5 from pathlib import Path
      6 

ImportError: cannot import name 'TypedDict' from 'typing' (/opt/conda/lib/python3.7/typing.py)

possible fix is

import sys

if sys.version_info >= (3, 8):
    from typing import TypedDict, Literal, overload  # pylint: disable=no-name-in-module
else:
    from typing_extensions import TypedDict, Literal, overload