NAS-140909 / 27.0.0-BETA.1 / remove isodate dependency

[yocalebo]

Replaces the isodate third-party package with builtin Python functionality. The package was used in only three files for two functions, both easily covered by the standard library now that the project requires Python 3.11+.

• Added a small parse_duration helper in zettarepl/utils/datetime.py. It accepts ISO 8601 durations restricted to weeks, days, hours, minutes, and seconds, which is the full set of components representable as a timedelta and the only set ever used in this codebase.
• Replaced isodate.parse_duration calls in zettarepl/snapshot/task/task.py and zettarepl/replication/task/retention_policy.py with the new helper.
• Replaced isodate.parse_time calls in zettarepl/scheduler/cron.py with the stdlib datetime.time.fromisoformat, which handles every begin/end value the project uses.
• Dropped isodate from setup.py and removed the matching mypy override from pyproject.toml.

The primary caller is the TrueNAS middleware. In middlewared/plugins/zettarepl.py, lifetime_iso8601 always emits durations of the form PT<int>S. Even MONTH and YEAR units are pre-flattened to days at the middleware layer (30 and 365 days respectively) before being formatted, so ISO year and month components never reach zettarepl. The begin and end schedule values are likewise emitted as plain HH MM strings.

The existing lifetime type annotation is timedelta, which means year and month durations were already implicitly unsupported. Sorting them in CustomSnapshotRetentionPolicy would have failed because isodate.Duration does not compare with timedelta. The new helper rejects those components explicitly rather than silently producing a value that breaks downstream code.

The hand-written YAML configs that appear in the integration tests use only P1D, P14D, P30D, P7W, and PT1H, all of which the new parser handles.

• All 145 unit tests pass.
• mypy --strict reports no issues on the four changed source files.

[bugclerk]

Jira URL: https://ixsystems.atlassian.net/browse/NAS-140909

[yocalebo]

time 00:45

[bugclerk]

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