Split dev-facing extras into PEP 735 dependency groups (and the pip ≥ 25.1 implication)

THis will keep linopy up to date and the distributed package cleaner. Does any contributor use pip <25.1 and cant upgrade?

[!NOTE]
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Summary

Today every optional dependency set lives in [project.optional-dependencies] (extras): oetc, remote, docs, dev, solvers. We have no [dependency-groups].

Some of these are purely dev/contributor-facing (dev, docs, plus a benchmarks set) and have no business in the published wheel metadata. PEP 735 [dependency-groups] is the right home for them; runtime-facing extras consumers install from PyPI (solvers, oetc, remote) should stay as extras.

Proposal

Move the dev/contributor-only sets out of [project.optional-dependencies] and into [dependency-groups]. Keep user-facing runtime extras where they are.

Implication for pip users (the part that needs a heads-up)

Dependency groups are not extras, and pip treats them differently:

• The split is effectively a breaking change for contributors on pip < 25.1 — older pip has no support for groups at all.
• Groups are not reachable via the .[...] syntax (pip install ".[dev]" would stop working for anything that moved), and they aren't published in wheel metadata.

So the move needs a documented pip path (in doc/contributing.rst) and a clear note about the pip ≥ 25.1 requirement, alongside the existing uv instructions.

Inspect the project metadata where the current optional dependency sets are defined, separating dev, docs, and benchmark dependencies from the runtime extras. Read doc/contributing.rst and the existing uv instructions first. Done means contributor installation is documented for pip ≥25.1 and uv, the runtime extras remain available, and the moved groups no longer depend on the old extras syntax.