feat(py client): read from DELPHI_EPIDATA_KEY

[dshemetov]

This should make CI integrations easier, by allowing us to set the env var to a secret. Also mimics behavior in epidatr.

Summary:

• read API key from DELPHI_EPIDATA_KEY env var by default, else None

Prerequisites:

• Unless it is a documentation hotfix it should be merged against the dev branch
• Branch is up-to-date with the branch to be merged with, i.e. dev
• Build is successful
• Code is cleaned up and formatted

[sonarcloud]

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0.0% Duplication

[krivard]

see #1232 (review)

[melange396]

if the motivation is to be able to provide a secret API Key for CI tests, theres no need: those tests dont use production data or services, and we can insert any old "test" key into the local test database during the pre-test setups. (like here)

for other purposes, id shy away from using the environment variable for the API Key... imnsho, users of the python client library should assign the key in their code, or they can pull the value from something external themselves.

[melange396]

its not obvious (though it should be made clearer (probbly with a setter method)), but this variable should be assigned a 2-tuple of the username "epidata" followed by the key (or the var can be set to a different Auth-like object, but thats a longer story) -- a plain string will not work. see covidcast/covidcast.py for a reference.

[dshemetov]

ah, yea Minh and I ran into this earlier

[dshemetov]

No need