Merge pyodide_py into python_stdlib.zip. ### Description

Merges pyodide_py.tar into python_stdlib.zip. This reduces the number of network requests during the initialization.

The original intention of splitting pyodide_py.tar from pyodide.asm.data was to give users an option to provide custom files (#2023), which is now available by modifying python_stdlib.zip directly. So I think there is no reason to have both python_stdlib.zip and pyodide_py.tar.

### Checklists

- [x] Add a [CHANGELOG](https://github.com/pyodide/pyodide/blob/main/docs/project/changelog.md) entry
- [x] Add / update tests

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coverage:
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    project:
      default:
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        # project coverage decrease by more than 1%:
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    patch:
      default:
        # Be tolerant on slight code coverage diff on PRs to limit
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        # Note: The coverage stats are still uploaded
        # to codecov so that PR reviewers can see uncovered lines
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codecov:
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    # to avoid confusion.
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