Improvements to Emscripten/Pyodide CI

[hoodmane]

Since we released new Pyodide alpha, we can clean up the CI job a lot.

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[hoodmane]

On my local build I get:

💥 maturin failed
  Caused by: Invalid PYO3_CONFIG_FILE
  Caused by: missing value for ext_suffix

but if I put ext_suffix into the PYO3_CONFIG_FILE it says it shouldn't be there. @messense @davidhewitt?

[messense]

but if I put ext_suffix into the PYO3_CONFIG_FILE it says it shouldn't be there

#897 (comment)

https://github.com/PyO3/pyo3/blob/3fd0c0e1421c71b0e5755427eaaa886cc67cd9c1/pyo3-build-config/src/impl_.rs#L436

This is a bug, I guess we can strip maturin specific config keys before passing to pyo3.

[messense]

Thanks!