Fails on Python 3.7, can't import Literal from typing

[ahwillia]

Recently had to downgrade Python to 3.7 for some reason and ran into this.

---------------------------------------------------------------------------
ImportError                               Traceback (most recent call last)

~/code/netrep/netrep/metrics/__init__.py in <module>
      1 from netrep.metrics.cka import LinearCKA
----> 2 from netrep.metrics.linear import LinearMetric
      3 from netrep.metrics.perm import PermutationMetric
      4 from netrep.metrics.stochastic import GaussianStochasticMetric
      5 from netrep.metrics.stochastic import EnergyStochasticMetric

~/code/netrep/netrep/metrics/linear.py in <module>
      1 from __future__ import annotations
----> 2 from typing import Literal, Tuple
      3 
      4 import numpy as np
      5 import numpy.typing as npt

ImportError: cannot import name 'Literal' from 'typing' (/mnt/home/awilliams/anaconda3/envs/allensdk/lib/python3.7/typing.py)

We should specify our requirements better, or figure out a way to support Python 3.7

[talmo]

You'll need to add typing_extensions to your deps, and then:

try:
    from typing import Literal
except ImportError:
    from typing_extensions import Literal

Ref: https://stackoverflow.com/a/67193166/1939934