Openshift - datetime value handling

[Niloth-p]

We want to use only ISO 8601 formats with the <time:> syntax, so we remove the previous comment recommending the usage of <time:dep_time>, and instead add a (commented-out) function that converts the datetime to the right format.

How did you test this PR?
I did not.

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[Niloth-p]

Updates:

• Added a datetime_to_global_time helper in zulip/zulip/__init__.py.
• Uncommented the function, and edited the comment above it.

This version still uses strptime(). Since OpenShift's datetime strings use a space instead of the "T".
I have another branch with a version that uses fromisoformat() (the backported one) here just in case.

[Niloth-p]

Updates:

• Removed the string replacement
• Removed __all__

Apologies for the embarrasing errors. I even checked that docs page earlier. 🤷
Thank you for the patient review, and the help with the re-export, @andersk.

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Tangential questions (if you have the time):

• Ruff's doc on PLC0414 says that rule does not apply in __init__.py files. But, it still raised that error, not sure if I'm misunderstanding it.
• Weirdly, the previous GitHub CI runs that ended with mypy errors did not raise any issues when locally running .tools/lint. What could be causing that mismatch? Shouldn't the activated venv be enough to make the results uniform?

[timabbott]

Looks good to me, merged, thanks @Niloth-p!

I encourage discussing those tangential questions in new #integrations threads in chat.zulip.org.