chore: unpin fsspec, bump to >=2026.2.0 with cloud extras

[asmithml]

Summary

• Bump fsspec from ==2025.7.0 to >=2026.2.0
• Bump s3fs and gcsfs from ==2025.7.0 to >=2026.2.0
• Add [project.optional-dependencies] with s3, gcs, and cloud extras for pip/uv users
• Update README with uv install instructions and fix dev commands (-e dev)

Why

The exact pin on fsspec==2025.7.0 (along with matching s3fs/gcsfs pins) was blocking downstream tools from upgrading dependencies. No documented reason for the pin exists, and the project only uses stable public APIs (AbstractFileSystem, HTTPFileSystem, LocalFileSystem) which have not changed across versions.

Validation

• All 79 tests pass on fsspec==2026.2.0
• Verified install paths for pip, uv, and pixi
• No code changes required, only dependency version constraints

Installer	fsspec	s3fs	gcsfs
pip	2026.3.0	2026.3.0	2026.3.0
uv	2026.3.0	2026.3.0	2026.3.0
pixi	2026.2.0	2026.2.0	2026.2.0

cc @m-mohr

[asmithml]

Reproducing install verification

To verify all three install methods resolve correctly, clone this branch and run:

pip

python -m venv .venv && source .venv/bin/activate
pip install -e . && python -c "import fsspec; print(fsspec.__version__)"
pip install -e ".[cloud]" && python -c "import s3fs, gcsfs; print(s3fs.__version__, gcsfs.__version__)"

uv

uv venv && source .venv/bin/activate
uv pip install -e . && python -c "import fsspec; print(fsspec.__version__)"
uv pip install -e ".[cloud]" && python -c "import s3fs, gcsfs; print(s3fs.__version__, gcsfs.__version__)"

pixi

pixi install -e all
pixi run -e all python -c "import fsspec, s3fs, gcsfs; print(fsspec.__version__, s3fs.__version__, gcsfs.__version__)"

All versions should be >=2026.2.0. pip/uv pull from PyPI, pixi from conda-forge.