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- [x] Add new / update outdated documentation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: micropip
  # This version needs to be consistent with docs/requirements-doc.txt
  version: "0.2.0"
  tag:
    - always
  top-level:
    - micropip

source:
  sha256: be2914a08e553e6d05cacde692db00412e6fc508d63ff70e2ae94050149c82d6
  url: https://files.pythonhosted.org/packages/a1/7c/2f5a9b925a66261b39a9c4181c9cc4500ff5547d7b1efc76f5cba40a1d0e/micropip-0.2.0-py3-none-any.whl

requirements:
  run:
    - pyparsing
    - packaging

package:
  name: micropip
  # This version needs to be consistent with docs/requirements-doc.txt
  version: "0.2.0"
  tag:
    - always
  top-level:
    - micropip

source:
  sha256: be2914a08e553e6d05cacde692db00412e6fc508d63ff70e2ae94050149c82d6
  url: https://files.pythonhosted.org/packages/a1/7c/2f5a9b925a66261b39a9c4181c9cc4500ff5547d7b1efc76f5cba40a1d0e/micropip-0.2.0-py3-none-any.whl

requirements:
  run:
    - pyparsing
    - packaging
