Fix incorrect padding in the access token expiration datetime

[bachya]

Describe what the PR does:

My logic for storing the access token's expiration datetime (appropriately padded) was incorrect. This could result in multiple successful (but unnecessary) requests to refresh the access token if those requests are within microseconds of each other. This PR fixes the logic.

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.

[qlty-cloud-legacy]

Code Climate has analyzed commit b976234 and detected 0 issues on this pull request.

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[codecov]

Codecov Report

Merging #294 (b976234) into dev (3167e43) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##              dev     #294   +/-   ##
=======================================
  Coverage   99.77%   99.77%           
=======================================
  Files          13       13           
  Lines         903      903           
=======================================
  Hits          901      901           
  Misses          2        2           

Impacted Files	Coverage Δ
simplipy/api.py	98.48% <100.00%> (ø)

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