Ensure that the refresh token is only used within the expiration window

[bachya]

Describe what the PR does:

Previously, when a request would encounter a 401 backoff, the library would attempt to refresh the access token for each of those requests, often leading to collisions where the refresh token would become invalid. This PR ensures that backoff will only try the refresh token if the last time it was used is outside of the expiration window.

Does this fix a specific issue?

N/A

Checklist:

• Confirm that one or more new tests are written for the new functionality.
• Run tests and ensure everything passes (with 100% test coverage).
• Update README.md and docs/ with any new documentation.
• Add yourself to AUTHORS.md.

[codecov]

Codecov Report

Merging #266 (b803fa8) into dev (a8650ef) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               dev      #266   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           13        13           
  Lines          886       893    +7     
=========================================
+ Hits           886       893    +7     

Impacted Files	Coverage Δ
simplipy/api.py	100.00% <100.00%> (ø)

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