refactor: remove iso8601

[waketzheng]

Description

Library iso8601 does not update for a long time

Motivation and Context

Reduce dependency suggested by #1956

How Has This Been Tested?

make ci

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added the changelog accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codspeed-hq]

CodSpeed Performance Report

Merging #2024 will not alter performance

_{Comparing waketzheng:remove-iso8601 (02776d8) with develop (752762c)}

Summary

✅ 16 untouched

[coveralls]

Pull Request Test Coverage Report for Build 19801808583

Details

• 2 of 2 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.001%) to 90.716%

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Totals
Change from base Build 19726043557:	-0.001%
Covered Lines:	6630
Relevant Lines:	7194

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💛 - Coveralls