Add support for Python 3.15 and fix deprecation warnings

[hugovk]

Description

The Python 3.15 beta is out. 🚀

The 3.15 release manager (👋 hi, that's me!) said:

We strongly encourage maintainers of third-party Python projects to test with 3.15 during the beta phase and report issues found to the Python bug tracker as soon as possible. While the release is planned to be feature-complete entering the beta phase, it is possible that features may be modified or, in rare cases, removed up until the start of the release candidate phase (2026-08-04). Our goal is to have no ABI changes after beta 4 and as few code changes as possible after the first release candidate. To achieve that, it will be extremely important to get as much exposure for 3.15 as possible during the beta phase.

This includes creating pre-release wheels for 3.15, as it helps other projects to do their own testing. However, we recommend that your regular production releases wait until 3.15.0rc1, to avoid the risk of ABI breaks.

Also fix PendingDeprecationWarning: FileType is deprecated. Simply open files after parsing arguments. starting in Python 3.14, and bump pluggy in the lockfile to fix SyntaxWarning: 'return' in a 'finally' block.

Checklist

• Tests covering the new functionality have been added
• Documentation has been updated OR the changes are too minor to be documented
• The Changes are listed in the CHANGELOG.md OR the changes are insignificant