Update datetime requirement from ~=5.5 to ~=6.0

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Updates the requirements on datetime to permit the latest version.

Changelog

Sourced from datetime's changelog.

6.0 (2025-11-25)

• 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.

5.5 (2024-03-21)

• Change pickle format to export the microseconds as an int, to solve a problem with dates after 2038. ([#56](https://github.com/zopefoundation/DateTime/issues/56) <https://github.com/zopefoundation/DateTime/issues/56>_)

5.4 (2023-12-15)

• Fix UnknownTimeZoneError when unpickling DateTime.DateTime().asdatetime(). ([#58](https://github.com/zopefoundation/DateTime/issues/58) <https://github.com/zopefoundation/DateTime/issues/58>_)

• Repair equality comparison between DateTime instances and other types. ([#60](https://github.com/zopefoundation/DateTime/issues/60) <https://github.com/zopefoundation/DateTime/issues/60>_)

5.3 (2023-11-14)

• Add support for Python 3.12.

• Add preliminary support for Python 3.13a2.

5.2 (2023-07-19)

• Cast int to float in compare methods.
• Fix compare methods between DateTime instances and None. ([#52](https://github.com/zopefoundation/DateTime/issues/52) <https://github.com/zopefoundation/DateTime/issues/52>_)

5.1 (2023-03-14)

• Add missing python_requires to setup.py.

... (truncated)

Commits

• f880a58 Preparing release 6.0
• c5d41db Prepare release.
• d53bac4 Update Python version support. (#69)
• b8f078c Update Python version support. (#68)
• d67044d Add missing version specifier to setup.py
• e95b160 Merge pull request #66 from zopefoundation/config-with-zope-product-template-...
• 28d6788 - remove exclude
• 77babde fixup! - update to latest meta/config templates (#67)
• c6d71aa - update to latest meta/config templates
• e0c8be8 Merge pull request #65 from zopefoundation/setuptools-fossils
• Additional commits viewable in compare view

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