using CLI errors out for version 2.1.x

[jreiberkyle]

Expected behavior
on the latest version of the planet package (2.1.x), planet results in the CLI help being displayed

Actual behavior (describe the problem)
planet results in the following traceback

Traceback (most recent call last):
  File "/Users/jennifer.kyle/.pyenv/versions/planet-client-python-3.7.9/bin/planet", line 5, in <module>
    from planet.cli.cli import main
  File "/Users/jennifer.kyle/.pyenv/versions/planet-client-python-3.7.9/lib/python3.7/site-packages/planet/__init__.py", line 16, in <module>
    from . import data_filter, order_request, reporting, subscription_request
  File "/Users/jennifer.kyle/.pyenv/versions/planet-client-python-3.7.9/lib/python3.7/site-packages/planet/subscription_request.py", line 16, in <module>
    from typing import Any, Dict, Optional, List, Literal, Mapping, Sequence
ImportError: cannot import name 'Literal' from 'typing' (/Users/jennifer.kyle/.pyenv/versions/3.7.9/lib/python3.7/typing.py)

Workaround
pin install to 2.0.x with pip install planet==2.0

Installation Method
pip install planet
and pip install -e . run from repo root dir result in the same behavior.

[jreiberkyle]

maybe this boils down to changing the min version of python for the package at

planet-client-python/setup.py

Line 79 in 74e8b77

python_requires='>=3.7',

nvm, looks like with #998 3.7 support will be restored