fix: rename period to interval in repr and tests

[Benji19967]

Hi @sdispater @Secrus!

This PR renames the remaining mentions of period/Period to interval/Interval, where it makes sense.

• Rename Period to Interval in
  • Interval.__repr__
  • ValueError message of Interval.__new__
  • docstrings of some methods in Interval
• Rename period to interval (mostly in tests) for a more consistent codebase. I left the docs untouched.

A quick rg search will show the only mentions left of period/Period are in docs/, formatting/, locales/, CHANGELOG.md or clock

PS: Thanks for the great library! I have relied on it a lot, both for professional and personal projects.
PPS: I have some free time on my hands in the next couple of months and would happily contribute more, if you guys have any suggestions.

Pull Request Check List

• Added Updated tests for changed code.
• Updated documentation for changed code.

[codspeed-hq]

CodSpeed Performance Report

Merging #757 will degrade performances by 19.12%

_{Comparing Benji19967:master (997999a) with master (18f9c0e)}

Summary

❌ 1 regressions
✅ 1 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	master	Benji19967:master	Change
❌	test_format	2.3 ms	2.8 ms	-19.12%