Chore: consistent metadata in wheels

[dimbleby]

I do not fully understand why pipcl and pytest become project dependencies on pyodide only, but the best way to express that is through environment markers. Else, wheels uploaded to pypi contain different metadata.

Cross-platform resolvers like uv and poetry typically just look at one wheel and trust that all have the same requirements: so depending on which they happen to look at: you are either getting pipcl and pytest always installed, or never installed.

Signaling the requirement through static metadata allows the right answer to be reached on all platforms.

[github-actions]

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[dimbleby]

I will not be signing a CLA, you are welcome to this code, take it or leave it