fix(deps): update dependency datetime to v6 (#398)

This PR contains the following updates:

| Package | Change | Age | Confidence |
|---|---|---|---|
| [datetime](https://github.com/zopefoundation/DateTime) | `<6.0,>=5.5` -> `<6.1,>=6` | [![age](https://developer.mend.io/api/mc/badges/age/pypi/datetime/6.0?slim=true)](https://docs.renovatebot.com/merge-confidence/) | [![confidence](https://developer.mend.io/api/mc/badges/confidence/pypi/datetime/5.5/6.0?slim=true)](https://docs.renovatebot.com/merge-confidence/) |

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### Release Notes

<details>
<summary>zopefoundation/DateTime (datetime)</summary>

### [`v6.0`](https://github.com/zopefoundation/DateTime/blob/HEAD/CHANGES.rst#60-2025-11-25)

[Compare Source](https://github.com/zopefoundation/DateTime/compare/5.5...6.0)

- Add support for Python 3.13, 3.14.

- Drop support for Python 3.7, 3.8, 3.9.

- Remove setuptools fossils.

- Remove Python2 compatiblity code.

</details>

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### Configuration

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Reviewed-on: #398
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Co-committed-by: renovate[bot] <renovate-bot@git.tainton.uk>

pyproject.toml

@@ -9,7 +9,7 @@ authors = [
 requires-python = "<3.14,>=3.11"
 dependencies = [
     "webex-bot<1.1.0,>=1.0.3",
-    "datetime<6.0,>=5.5",
+    "datetime<6.1,>=6",
     "requests<3.0.0,>=2.32.3",
 ]