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- [x] Add new / update outdated documentation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: pandas
  version: 1.5.2
  tag:
    - min-scipy-stack
  top-level:
    - pandas
source:
  url: https://files.pythonhosted.org/packages/4d/07/c4d69e1acb7723ca49d24fc60a89aa07a914dfb8e7a07fdbb9d8646630cd/pandas-1.5.2.tar.gz
  sha256: 220b98d15cee0b2cd839a6358bd1f273d0356bf964c1a1aeb32d47db0215488b
build:
  cflags:
    -Werror=implicit-function-declaration -Werror=mismatched-parameter-types
    -Werror=mismatched-return-types -Oz
  ldflags: --Wl,--fatal-warnings -Oz
requirements:
  host:
    - numpy
  run:
    - distutils
    - numpy
    - python-dateutil
    - pytz
    - setuptools
about:
  home: https://pandas.pydata.org
  PyPI: https://pypi.org/project/pandas
  summary: Powerful data structures for data analysis, time series, and statistics
  license: BSD-3-Clause

package:
  name: pandas
  version: 1.5.2
  tag:
    - min-scipy-stack
  top-level:
    - pandas
source:
  url: https://files.pythonhosted.org/packages/4d/07/c4d69e1acb7723ca49d24fc60a89aa07a914dfb8e7a07fdbb9d8646630cd/pandas-1.5.2.tar.gz
  sha256: 220b98d15cee0b2cd839a6358bd1f273d0356bf964c1a1aeb32d47db0215488b
build:
  cflags:
    -Werror=implicit-function-declaration -Werror=mismatched-parameter-types
    -Werror=mismatched-return-types -Oz
  ldflags: --Wl,--fatal-warnings -Oz
requirements:
  host:
    - numpy
  run:
    - distutils
    - numpy
    - python-dateutil
    - pytz
    - setuptools
about:
  home: https://pandas.pydata.org
  PyPI: https://pypi.org/project/pandas
  summary: Powerful data structures for data analysis, time series, and statistics
  license: BSD-3-Clause
