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- [x] Add new / update outdated documentation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: scikit-image
  version: 0.19.3
  top-level:
    - skimage
    # - doc  # scikit-image has top level `doc` directory... but it is too common name so we don't want to use it as a hint for top-level import.
source:
  patches:
    - patches/preferred_plugins.patch
    - patches/make-tifffile-optional.patch
  sha256: 24b5367de1762da6ee126dd8f30cc4e7efda474e0d7d70685433f0e3aa2ec450
  url: https://files.pythonhosted.org/packages/00/d4/6682033d02917b10a2024dbe5a0636d2338b0799f7bd1885508fb114aec9/scikit-image-0.19.3.tar.gz
requirements:
  host:
    - numpy
  run:
    - distutils
    - packaging
    - numpy
    - scipy
    - matplotlib
    - networkx
    - Pillow
    - imageio
    - pywavelets

about:
  home: https://scikit-image.org
  PyPI: https://pypi.org/project/scikit-image
  summary: Image processing in Python
  license: Modified BSD

package:
  name: scikit-image
  version: 0.19.3
  top-level:
    - skimage
    # - doc  # scikit-image has top level `doc` directory... but it is too common name so we don't want to use it as a hint for top-level import.
source:
  patches:
    - patches/preferred_plugins.patch
    - patches/make-tifffile-optional.patch
  sha256: 24b5367de1762da6ee126dd8f30cc4e7efda474e0d7d70685433f0e3aa2ec450
  url: https://files.pythonhosted.org/packages/00/d4/6682033d02917b10a2024dbe5a0636d2338b0799f7bd1885508fb114aec9/scikit-image-0.19.3.tar.gz
requirements:
  host:
    - numpy
  run:
    - distutils
    - packaging
    - numpy
    - scipy
    - matplotlib
    - networkx
    - Pillow
    - imageio
    - pywavelets

about:
  home: https://scikit-image.org
  PyPI: https://pypi.org/project/scikit-image
  summary: Image processing in Python
  license: Modified BSD
