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- [x] Add new / update outdated documentation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: Pygments
  version: 2.13.0
  top-level:
    - pygments
source:
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  url: https://files.pythonhosted.org/packages/4f/82/672cd382e5b39ab1cd422a672382f08a1fb3d08d9e0c0f3707f33a52063b/Pygments-2.13.0-py3-none-any.whl
about:
  home: https://pygments.org/
  PyPI: https://pypi.org/project/Pygments
  summary: Pygments is a syntax highlighting package written in Python.
  license: BSD License

package:
  name: Pygments
  version: 2.13.0
  top-level:
    - pygments
source:
  sha256: f643f331ab57ba3c9d89212ee4a2dabc6e94f117cf4eefde99a0574720d14c42
  url: https://files.pythonhosted.org/packages/4f/82/672cd382e5b39ab1cd422a672382f08a1fb3d08d9e0c0f3707f33a52063b/Pygments-2.13.0-py3-none-any.whl
about:
  home: https://pygments.org/
  PyPI: https://pypi.org/project/Pygments
  summary: Pygments is a syntax highlighting package written in Python.
  license: BSD License
