[py] Support Python 3.15

[cgoldberg]

supersedes: #17502

💥 What does this PR do?

Python 3.15 ships Oct. 1, 2026 Oct. 9, 2026 (pushed back a week).

This PR adds support for Python 3.15 to the selenium python package:

Build/Tooling:

• updates rules_python for Bazel to version 2.4.1
• bumps all python toolchain versions to latest
• adds toolchain for 3.15

Packaging:

• Adds trove classifier for 3.15 to package metadata

Tests:

• updates CI to run unit tests against 3.15 instead of 3.14

Note: This PR uses rules_python 2.4.1, which currently has support for Python 3.15.0rc3. Once the final version of 3.15.0 is released, we should bump rules_python to the latest version and change the toolchain to use 3.15.0 instead of the release candidate.

🤖 AI assistance

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  • Tool(s):
  • What was generated:
  • I reviewed all AI output and can explain the change

🔄 Types of changes

• Build/Packaging

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[cgoldberg]

tests are failing with Python 3.10 on Windows?

ImportError: DLL load failed while importing _cffi_backend: The filename or extension is too long.

This is coming from the trio dependency... need to investigate why this doesn't work with new rules_python.

[titusfortner]

@cgoldberg I started down this rabbit hole and decided to back out of it, see my comment here: #17502 (comment)

[cgoldberg]

I started down this rabbit hole

... well if that isn't a total PITA. I'll try shortening some names to stay under the length limit.

[titusfortner]

@cgoldberg should we also remove support for 3.10 now since it is EOL? (I may have asked this elsewhere, I can't find the comment I thought I made already)

[cgoldberg]

@titusfortner if we follow what we did last time for 3.14 release, we would drop support for the oldest version since it is EOL.

However, all of our dependencies work with 3.10 and it wouldn't be a big deal to continue supporting it. We only really need to drop support once rules_python does (I think they support back to 3.9 atm) or if we run into dependency issues.

So, I'm fine with dropping 3.10 now or waiting a while. I think the only users that might be affected would be people on older Linux distros or working on some older legacy projects. I don't think we got a single complaint when we dropped 3.9.

[titusfortner]

Yeah, I'm generally in favor of removing unsupported versions of things quickly allowing us to use new features where possible.

[cgoldberg]

@titusfortner I am going to create a new PR once the final version of 3.15 ships and is included in rules_python (hopefully later today). I'll drop 3.10 support then also.