Releasing 1.3.4 or 1.4.0 soon?

There hasn't been a release in quite a while, 1.3.3 is from November 2025, as pointed out in https://github.com/explosion/cython-blis/issues/120#issuecomment-5422544270 recently. Those contributors need `win-arm64` wheels.

There are a number of fixes and improvements in `main`:

- Free-threaded Python support
- `win-arm64` wheels
- Build fixes for `ppc64le` (POWER8/POWER9)
- Build fixes for recent GCC and LLVM compilers and AVX-512
- Support for parallel builds
- A host of test suite improvements and cleanups

The one thing that needs doing, while we're at it:

- Add Python 3.15 wheels

There are no open PRs that are important/blocking right now. I wanted to finish off gh-156, but that's just about build warnings - not critical.

@honnibal WDYT about adding 3.15 wheels and doing a release? What should the release number be?

Start by reviewing the changes already on main, especially the existing wheel support and the Python 3.15 requirement. Check gh-156 only for its build-warning scope, then determine the work needed for Python 3.15 wheels and settle whether the release should be 1.3.4 or 1.4.0. Done means the wheels are available and the release includes the listed main-branch fixes.

Written by the indexing model from the issue text.