Add support for AWS_SESSION_TOKEN and AWS_SECURITY_TOKEN

[bxm156]

Add support for reading the AWS_SESSION_TOKEN and AWS_SECURITY_TOKEN from the environment.

Verified this works as expected in AWS Lambda.

[bxm156]

A few questions:

1. Should this be behind a setting ?
2. Should we care if the there is a AWS_SESSION_TOKEN but no AWS_ACCESS/SECRET keys in the env?

[codecov-io]

Codecov Report

Merging #283 into master will increase coverage by 0.2%.
The diff coverage is 91.66%.

@@            Coverage Diff            @@
##           master     #283     +/-   ##
=========================================
+ Coverage   72.28%   72.48%   +0.2%     
=========================================
  Files          10       10             
  Lines        1371     1381     +10     
=========================================
+ Hits          991     1001     +10     
  Misses        380      380

Impacted Files	Coverage Δ
storages/backends/s3boto.py	86.36% <91.66%> (+0.22%)	⬆️
storages/backends/s3boto3.py	84.95% <91.66%> (+0.22%)	⬆️

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[AusIV]

I just encountered the same issue, and was glad to see someone had already done the legwork. I've also verified that this solves my problem on Lambda.

[narfman0]

I've verified this in my project as well, thanks Bryan! Should fix #282 too.

[jschneier]

Looks good!

[jschneier]

@bxm156 re: your questions. The docs are overdue for an overhaul. Authentication should be given its own section. I think relying on just the environment variables is likely better in this case but I need to read the aws docs again just to make sure.

[jschneier]

Oof. This only sets .security_token if it exists. I need to switch to live tests because you always try to reference the parameter even if it isn't getting set.

[jschneier]

Crappy fix in 2529729

[bxm156]

Ah, sorry about that, I was planning on writing some tests for this but never got around to it :(