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- [x] Add new / update outdated documentation

#!/bin/bash

# get the absolute path of the root folder
# shellcheck disable=SC2164
ROOT=$(cd -- "$(dirname "${BASH_SOURCE[0]}")" >/dev/null 2>&1 ; pwd -P)

# emsdk_env.sh is fairly noisy, and suppress error message if the file doesn't
# exist yet (i.e. before building emsdk)
# shellcheck source=/dev/null
source "$ROOT/emsdk/emsdk/emsdk_env.sh" 2> /dev/null || true
export PATH="$ROOT/node_modules/.bin/:$ROOT/emsdk/emsdk/ccache/git-emscripten_64bit/bin:$PATH:$ROOT/packages/.artifacts/bin/"
EMCC_PATH=$(which emcc.py || echo ".")
EM_DIR=$(dirname "$EMCC_PATH")
export EM_DIR

# Following two variables are set by emsdk activated otherwise
export _EMCC_CCACHE=1
# mtime of this file is checked by ccache, we set it to avoid cache misses.
export EM_CONFIG="$ROOT/emsdk/emsdk/.emscripten"

#!/bin/bash

# get the absolute path of the root folder
# shellcheck disable=SC2164
ROOT=$(cd -- "$(dirname "${BASH_SOURCE[0]}")" >/dev/null 2>&1 ; pwd -P)

# emsdk_env.sh is fairly noisy, and suppress error message if the file doesn't
# exist yet (i.e. before building emsdk)
# shellcheck source=/dev/null
source "$ROOT/emsdk/emsdk/emsdk_env.sh" 2> /dev/null || true
export PATH="$ROOT/node_modules/.bin/:$ROOT/emsdk/emsdk/ccache/git-emscripten_64bit/bin:$PATH:$ROOT/packages/.artifacts/bin/"
EMCC_PATH=$(which emcc.py || echo ".")
EM_DIR=$(dirname "$EMCC_PATH")
export EM_DIR

# Following two variables are set by emsdk activated otherwise
export _EMCC_CCACHE=1
# mtime of this file is checked by ccache, we set it to avoid cache misses.
export EM_CONFIG="$ROOT/emsdk/emsdk/.emscripten"
