S3Storage - pull AWS_SESSION_TOKEN from environment

[samdoolin]

S3Storage looks for AWS_ACCESS_KEY_ID and AWS_SECRET_ACCESS_KEY in settings and in os.environ.

django-storages/storages/backends/s3.py

Line 366 in d84ad92

"access_key": setting(

However it only looks for AWS_SESSION_TOKEN in settings but not in os.environ.

django-storages/storages/backends/s3.py

Line 380 in d84ad92

"security_token": setting(

All 3 are used to initialise the the boto3.Session.

django-storages/storages/backends/s3.py

Line 457 in d84ad92

session = boto3.Session(

Please consider allowing security_token to fall back to os.environ as well?
Otherwise I need to manually wire AWS_SESSION_TOKEN into settings to avoid url failures (InvalidAccessKeyId).
And/or describe how AWS_SESSION_TOKEN is handled in the docs.

Thanks

[jschneier]

Sure, please open a PR

[samdoolin]

does #1399 look ok?