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- [x] Add new / update outdated documentation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: sqlalchemy
  version: 1.4.45
  top-level:
    - sqlalchemy
source:
  url: https://files.pythonhosted.org/packages/76/d5/9ce70fd0d2858c72ecacff0c0518e9ddfbbaf4753b85e49f6d94ad74de36/SQLAlchemy-1.4.45.tar.gz
  sha256: fd69850860093a3f69fefe0ab56d041edfdfe18510b53d9a2eaecba2f15fa795
requirements:
  run:
    - sqlite3
about:
  home: https://www.sqlalchemy.org
  PyPI: https://pypi.org/project/sqlalchemy
  summary: Database Abstraction Library
  license: MIT

package:
  name: sqlalchemy
  version: 1.4.45
  top-level:
    - sqlalchemy
source:
  url: https://files.pythonhosted.org/packages/76/d5/9ce70fd0d2858c72ecacff0c0518e9ddfbbaf4753b85e49f6d94ad74de36/SQLAlchemy-1.4.45.tar.gz
  sha256: fd69850860093a3f69fefe0ab56d041edfdfe18510b53d9a2eaecba2f15fa795
requirements:
  run:
    - sqlite3
about:
  home: https://www.sqlalchemy.org
  PyPI: https://pypi.org/project/sqlalchemy
  summary: Database Abstraction Library
  license: MIT
