Retry 500 status code

[pl-gideon]

Is your feature request related to a problem? Please describe.
Requests occasionally will fail due to an intermittent internal server error. Retrying the request shortly afterwards is successful.

httpx.HTTPStatusError: Server error '500 Internal Server Error' for url 'https://api.planet.com/subscriptions/v1’

Describe the solution you'd like
Add 500 to list of status codes that are retried.

Describe alternatives you've considered
None

Additional context
None