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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  version: 6.5.0
  top-level:
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source:
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requirements:
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about:
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  PyPI: https://pypi.org/project/coverage
  summary: Code coverage measurement for Python
  license: Apache 2.0

package:
  name: coverage
  version: 6.5.0
  top-level:
    - coverage
source:
  url: https://files.pythonhosted.org/packages/5c/66/38d1870cb7cf62da49add1d6803fdbcdef632b2808b5c80bcac35b7634d8/coverage-6.5.0.tar.gz
  sha256: f642e90754ee3e06b0e7e51bce3379590e76b7f76b708e1a71ff043f87025c84
requirements:
  run:
    - sqlite3
about:
  home: https://github.com/nedbat/coveragepy
  PyPI: https://pypi.org/project/coverage
  summary: Code coverage measurement for Python
  license: Apache 2.0
