chore(deps): update dependency idna to v3.15

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
idna (changelog)	==3.14 → ==3.15

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Release Notes

kjd/idna (idna)

v3.15

Compare Source

• Enforce DNS-length cap on individual labels early in check_label,
  short-circuiting contextual-rule processing for oversized input
  while staying compatible with UTS 46 usage.
• Tidy core helpers: hoist bidi category sets to module-level
  frozensets (avoiding per-codepoint list construction), simplify
  length checks, and reuse the shared _unicode_dots_re from
  idna.core in the codec module.
• Use raise ... from err for proper exception chaining and
  switch internal string formatting to f-strings.
• Allow flit_core 4.x in the build backend.
• Expand the ruff lint set (flake8-bugbear, flake8-simplify,
  pyupgrade, perflint) and apply the surfaced fixes; pin lint CI
  to Python 3.14.
• Add Dependabot configuration for GitHub Actions.
• Convert README and HISTORY from reStructuredText to Markdown.
• Reference CVE-2026-45409 for the 3.14 advisory in place of the
  initial GHSA identifier.

Thanks to Felix Yan, Stan Ulbrych, and metsw24-max for
contributions to this release.

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