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- [x] Add new / update outdated documentation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: cftime
  version: 1.6.2
  top-level:
    - cftime
source:
  url: https://files.pythonhosted.org/packages/cd/db/c528f26ee2dbac1dc348189386c2df745770bb22c02542e281d60212cb13/cftime-1.6.2.tar.gz
  sha256: 8614c00fb8a5046de304fdd86dbd224f99408185d7b245ac6628d0276596e6d2

requirements:
  run:
    - numpy

about:
  home: https://github.com/Unidata/cftime
  PyPI: https://pypi.org/project/cftime
  summary: Time-handling functionality from netcdf4-python
  license: "License :: OSI Approved :: MIT License"

package:
  name: cftime
  version: 1.6.2
  top-level:
    - cftime
source:
  url: https://files.pythonhosted.org/packages/cd/db/c528f26ee2dbac1dc348189386c2df745770bb22c02542e281d60212cb13/cftime-1.6.2.tar.gz
  sha256: 8614c00fb8a5046de304fdd86dbd224f99408185d7b245ac6628d0276596e6d2

requirements:
  run:
    - numpy

about:
  home: https://github.com/Unidata/cftime
  PyPI: https://pypi.org/project/cftime
  summary: Time-handling functionality from netcdf4-python
  license: "License :: OSI Approved :: MIT License"
