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

from pytest_pyodide import run_in_pyodide


@run_in_pyodide(packages=["xarray"])
def test_xarray(selenium):
    import xarray as xr

    da = xr.DataArray(data=[[0, 1, 2], [3, 4, 5]], dims=("x", "y"))
    xr.testing.assert_equal(a=da.mean(), b=xr.DataArray(data=2.5))

from pytest_pyodide import run_in_pyodide


@run_in_pyodide(packages=["xarray"])
def test_xarray(selenium):
    import xarray as xr

    da = xr.DataArray(data=[[0, 1, 2], [3, 4, 5]], dims=("x", "y"))
    xr.testing.assert_equal(a=da.mean(), b=xr.DataArray(data=2.5))
