Delegate date strings formatting to datetime.isoformat()

[gonzalo-bulnes]

Status

Ready for review

Description of Changes

Closes #6257 (see Proposed plan section in the description).

Changes proposed in this pull request:

• Remove the custom date string format (that used to be the only one supported by the SecureDrop Client)
• Use ISO8061 via datetime.isoformat() instead (should be easier to use consistently too)

Testing

• Visual review: do these changes make sense given the context recapped below?

Then test here:

• Pay special attention to CI: do the tests changed here still pass?

And then check for surprises downstream:

• Run make dev here and make regenerate-sdk-cassettes in freedomofpress/securedrop-client/client. It should pass.
• Run make test in freedomofprses/securedrop-client/client. It should pass.

Deployment

Once this change has been released, the try/except logic in https://github.com/freedomofpress/securedrop-client/blob/7f7f60e67f7606708a6c9ef519c9657cd738be9e/client/securedrop_client/sdk/timestamps.py can be removed in favor of just datetime.fromisoformat().

Checklist

If you made changes to the server application code:

• Linting (make lint) and tests (make test) pass in the development container

[gonzalo-bulnes]

@legoktm I'm trying to gather again the context around this PR. It seems like one of the API responses contains timestamps without time zone information (specific error in CI). I believe that's a legit test suite error and that we want to fix the API response to conform to the new convention, does that seem right to you?

[cfm]

Tl;dr: This change does not have any side effects on time-zone support. It simply standardizes us to datetime.isoformat() and should be either approved or closed based whether we currently think that standardization is worthwhile.

We decided in today's backlog-pruning session that this was worth revisiting in a time-box to see if we could finalize it. Substantively all I've done is rebase this from develop. But to recap the context here:

1. securedrop currently sends UTC timestamps in a custom API_DATETIME_FORMAT.
2. With this change, securedrop will send UTC timestamps exclusively in datetime.isoformat().
3. securedrop-client, specifically the SDK, will already parse either (1) or (2). (The test plan above confirms this.)
4. After this change has been released, securedrop-client can drop the trial-and-error parsing in (3).