Fix expiration check

[dobosevych]

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Migrated test from #7109.
expires can be only float or datetime, so the test wasn't completely valid. Datetime serialization fixed in kombu

[codecov]

Codecov Report

Merging #7552 (55d700d) into master (9f891ee) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #7552   +/-   ##
=======================================
  Coverage   89.54%   89.54%           
=======================================
  Files         138      138           
  Lines       16799    16799           
  Branches     2458     2458           
=======================================
  Hits        15042    15042           
  Misses       1517     1517           
  Partials      240      240           

Flag	Coverage Δ
unittests	89.53% <100.00%> (ø)

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Impacted Files	Coverage Δ
celery/app/base.py	96.45% <100.00%> (ø)

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[lgtm-com]

This pull request introduces 1 alert and fixes 1 when merging 55d700d into 9f891ee - view on LGTM.com

new alerts:

• 1 for Unused import

fixed alerts:

• 1 for Module is imported with 'import' and 'import from'