Set date_format to ISO8601 to silence pandas 2.0 UserWarning on read_csv

[weiji14]

Description of proposed changes

Use pd.read_csv(..., date_format="ISO8601") to silence UserWarning: Could not infer format, so each element will be parsed individually, falling back to dateutil. To ensure parsing is consistent and as-expected, please specify a format..

Note that this requires pandas>=2.0 to work.

References:

• API deprecate date_parser, add date_format pandas-dev/pandas#50601
• https://pandas.pydata.org/pandas-docs/version/2.0/reference/api/pandas.read_csv.html
• https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.to_datetime.html?highlight=iso8601

Fixes #2480

Reminders

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• Add tests for new features or tests that would have caught the bug that you're fixing.
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• Write detailed docstrings for all functions/methods.
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Slash Commands

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specific operations. Supported slash commands are:

• /format: automatically format and lint the code
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[weiji14]

This date_format parameter only works with pandas>=2.0. Do we want to set a minimum version of pandas, or make things backwards compatible with pandas=1.x?

[seisman]

make things backwards compatible with pandas=1.x?

This would be better.

[weiji14]

Ok, compatibility with pandas=1.x and 2.x should be done in fa2a6d3 and 146a7f4. I've also changed ci_tests_legacy to run with pandas<2 as a check.