feat: make TzInfo instantiable without any arguments

[PrettyWood]

Change Summary

Allow TzInfo() to be instantiated without arguments, defaulting to UTC (0 seconds offset). This provides convenience for creating UTC timezones and satisfies the Python datetime documentation requirement for tzinfo subclasses to support no-argument construction.

ℹ pickle already worked via pyo3 automatic __reduce__() implementation, but this improves API usability and ensures full compliance with datetime.tzinfo requirements.

Related issue number

fix #1745

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

Selected Reviewer: @davidhewitt

[PrettyWood]

please review

[codspeed-hq]

CodSpeed Performance Report

Merging #1777 will not alter performance

_{Comparing tzinfo-no-arg (6888921) with main (1b3815c)}

Summary

✅ 159 untouched benchmarks

[davidhewitt]

Thanks!