Revert dateutil lock back to >= 2.7.0

[jadchaar]

Pull Request Checklist

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• 🧪 Added tests for changed code.
• 🛠️ All tests pass when run locally (run tox or make test to find out!).
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• 📚 Updated documentation for changed code.
• ⏩ Code is up-to-date with the master branch.

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Description of Changes

Revert dateutil lock back to >=2.7.0 based on feedback from @pganssle in https://github.com/arrow-py/arrow/pull/1006/files

[codecov]

Codecov Report

Merging #1041 (c1bee75) into master (3f056f3) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master     #1041   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           10        10           
  Lines         2166      2166           
  Branches       344       344           
=========================================
  Hits          2166      2166           

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[anishnya]

LGTM