#25304 Add support to python 3.15

Description

This PR adds support for Python 3.15 to the Python generators.

Python 3.15 was officially released on October 9, 2026.

Changes

Added Python 3.15 as a supported version.

Updated the Python generator configuration and documentation.

Updated the relevant generated samples.

Related issue

Closes #25303

PR checklist

• Read the contribution guidelines.
• Run the following to build the project and update samples:

  ./mvnw clean package || exit
  ./bin/generate-samples.sh ./bin/configs/*.yaml || exit
  ./bin/utils/export_docs_generators.sh || exit
  

  (For Windows users, please run the script in WSL)
  Commit all changed files.
  This is important, as CI jobs will verify all generator outputs of your HEAD commit as it would merge with master.
  These must match the expectations made by your contribution.
  You may regenerate an individual generator by passing the relevant config(s) as an argument to the script, for example ./bin/generate-samples.sh bin/configs/java*.
  IMPORTANT: Do NOT purge/delete any folders/files (e.g. tests) when regenerating the samples as manually written tests may be removed.
• If your PR is targeting a particular programming language, @mention the technical committee members, so they are more likely to review the pull request.

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Summary by cubic

Adds Python 3.15 support to the Python generator CI templates and regenerated sample clients, and fixes a pycryptodome breaking change in the HTTP signature tests. Closes #25303.

• Enables 3.15 in GitHub Actions, GitLab CI, and Travis configurations; the pydantic v1 workflows list it as commented-out for now.
• Trims trailing whitespace in the "Install" step name in two sample workflows.
• Updates test_http_signature.py to use pycryptodome's current key export API and accept either of the two encrypted-key error messages.