Unvendor pydoc_data and _pydecimal. ### Description

Removes / unvendors some python modules:

- Remove `_aix_support.py`, which is for supporting IBM AIX OS.

- Unvendor `_pydecimal.py`.
  - _pydecimal is a pure Python implementation of `decimal` module. [Importing `decimal` fallbacks](https://github.com/python/cpython/blob/main/Lib/decimal.py) to `_pydecimal` if the C-implementation `_decimal` is not available. In our case, _decimal is available, so _pydecimal will not be normally used. 

- Unvendor `pydoc_data`.
  - pydoc_data contains [a large (~700KB) dictionary](https://github.com/python/cpython/blob/main/Lib/pydoc_data/topics.py) for explaining python builtins. This is mostly used when `help("...")` is called.

### Checklists

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- [x] Add new / update outdated documentation

package:
  name: zlib
  version: 1.2.13

source:
  sha256: b3a24de97a8fdbc835b9833169501030b8977031bcb54b3b3ac13740f846ab30
  url: https://zlib.net/zlib-1.2.13.tar.gz

build:
  type: static_library
  script: |
    CFLAGS="-fPIC" emconfigure ./configure --prefix=${WASM_LIBRARY_DIR}
    emmake make install -j ${PYODIDE_JOBS:-3}

package:
  name: zlib
  version: 1.2.13

source:
  sha256: b3a24de97a8fdbc835b9833169501030b8977031bcb54b3b3ac13740f846ab30
  url: https://zlib.net/zlib-1.2.13.tar.gz

build:
  type: static_library
  script: |
    CFLAGS="-fPIC" emconfigure ./configure --prefix=${WASM_LIBRARY_DIR}
    emmake make install -j ${PYODIDE_JOBS:-3}
