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- [x] Add new / update outdated documentation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: h5py
  version: 3.7.0
  top-level:
    - h5py
source:
  url: https://files.pythonhosted.org/packages/c5/40/7cf58e6230f0e76699f011c6d293dd47755997709a303a4e644823f3a753/h5py-3.7.0.tar.gz
  sha256: 3fcf37884383c5da64846ab510190720027dca0768def34dd8dcb659dbe5cbf3
  patches:
    - patches/0001-Fix-incompatible-pointer-type.patch
    - patches/configure.patch

requirements:
  run:
    - numpy
    - pkgconfig
  host:
    - libhdf5

build:
  vendor-sharedlib: true
  script: |
    export HDF5_MPI=OFF
    export H5PY_SETUP_REQUIRES="0"
    export HDF5_VERSION=1.12.1
    export HDF5_DIR=${WASM_LIBRARY_DIR}

package:
  name: h5py
  version: 3.7.0
  top-level:
    - h5py
source:
  url: https://files.pythonhosted.org/packages/c5/40/7cf58e6230f0e76699f011c6d293dd47755997709a303a4e644823f3a753/h5py-3.7.0.tar.gz
  sha256: 3fcf37884383c5da64846ab510190720027dca0768def34dd8dcb659dbe5cbf3
  patches:
    - patches/0001-Fix-incompatible-pointer-type.patch
    - patches/configure.patch

requirements:
  run:
    - numpy
    - pkgconfig
  host:
    - libhdf5

build:
  vendor-sharedlib: true
  script: |
    export HDF5_MPI=OFF
    export H5PY_SETUP_REQUIRES="0"
    export HDF5_VERSION=1.12.1
    export HDF5_DIR=${WASM_LIBRARY_DIR}
