Ensure correct order of time attribute

[danielhuppmann]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Name of contributors Added to AUTHORS.rst
• Description in RELEASE_NOTES.md Added

Description of PR

This PR modifies how the values for the time attribute are retrieved, fixing a bug where the order may not have been correct.

Closes #722

[codecov]

Codecov Report

Merging #723 (9854cca) into main (a39fdca) will increase coverage by 0.0%.
The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #723   +/-   ##
=====================================
  Coverage   95.0%   95.0%           
=====================================
  Files         59      59           
  Lines       5998    6003    +5     
=====================================
+ Hits        5702    5707    +5     
  Misses       296     296           

Impacted Files	Coverage Δ
pyam/core.py	95.4% <ø> (ø)
tests/test_core.py	100.0% <100.0%> (ø)
tests/test_time.py	100.0% <100.0%> (ø)

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