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- [x] Add new / update outdated documentation

# Pyodide Python compatibility

## Python Standard library

Most of the Python standard library is functional, except for the modules
listed in the sections below. A large part of the CPython test suite passes except for
tests skipped in
[`src/tests/python_tests.yaml`](https://github.com/pyodide/pyodide/blob/main/src/tests/python_tests.yaml)
or via [patches](https://github.com/pyodide/pyodide/tree/main/cpython/patches).

### Optional modules

The following stdlib modules are unvendored by default,
in order to reduce initial download size of Python distribution.
You can load all unvendored stdlib modules
when initializing Pyodide with, `loadPyodide({ fullStdLib : true })`.
However this has a significant impact on the download size.
Instead, it is better to load individual modules as needed using
{any}`pyodide.loadPackage` or {any}`micropip.install`.

- distutils
- ssl
- lzma
- sqlite3
- test: it is an exception to the above, since it is not loaded even if `fullStdLib` is set to true.

#### Modules with limited functionality

- hashlib: Hash algorithms that are depending on OpenSSL are not available by default.
  See Python [hashlib documentation](https://docs.python.org/3/library/hashlib.html)
  for list of algorithms that are dependent on OpenSSL. If you need those algorithms,
  you need to call `pyodide.loadPackage('hashlib')` or `micropip.install('hashlib')`
  **before** importing hashlib.

### Removed modules

The following modules are removed from the standard library to reduce download size and
since they currently wouldn't work in the WebAssembly VM,

- curses
- dbm
- ensurepip
- idlelib
- lib2to3
- tkinter
- turtle.py
- turtledemo
- venv
- pwd

### Included but not working modules

The following modules can be imported, but are not functional due to the limitations of the WebAssembly VM:

- multiprocessing
- threading
- sockets

as well as any functionality that requires these.

# Pyodide Python compatibility

## Python Standard library

Most of the Python standard library is functional, except for the modules
listed in the sections below. A large part of the CPython test suite passes except for
tests skipped in
[`src/tests/python_tests.yaml`](https://github.com/pyodide/pyodide/blob/main/src/tests/python_tests.yaml)
or via [patches](https://github.com/pyodide/pyodide/tree/main/cpython/patches).

### Optional modules

The following stdlib modules are unvendored by default,
in order to reduce initial download size of Python distribution.
You can load all unvendored stdlib modules
when initializing Pyodide with, `loadPyodide({ fullStdLib : true })`.
However this has a significant impact on the download size.
Instead, it is better to load individual modules as needed using
{any}`pyodide.loadPackage` or {any}`micropip.install`.

- distutils
- ssl
- lzma
- sqlite3
- test: it is an exception to the above, since it is not loaded even if `fullStdLib` is set to true.

#### Modules with limited functionality

- hashlib: Hash algorithms that are depending on OpenSSL are not available by default.
  See Python [hashlib documentation](https://docs.python.org/3/library/hashlib.html)
  for list of algorithms that are dependent on OpenSSL. If you need those algorithms,
  you need to call `pyodide.loadPackage('hashlib')` or `micropip.install('hashlib')`
  **before** importing hashlib.

- decimal: The decimal module has C (\_decimal) and Python (\_pydecimal) implementations
  with the same functionality. The Python implementation is not available by default.
  If you need a Python implementation of decimal, you need to call
  `pyodide.loadPackage('pydecimal')` or `micropip.install('pydecimal')`,
  then explicitly import `_pydecimal`.

- pydoc: Help messages for Python builtins are not available by default
  in order to reduce the initial download size. You need to call
  `pyodide.loadPackage('pydoc_data')` or `micropip.install('pydoc_data')`
  to enable them.

### Removed modules

The following modules are removed from the standard library to reduce download size and
since they currently wouldn't work in the WebAssembly VM,

- curses
- dbm
- ensurepip
- idlelib
- lib2to3
- tkinter
- turtle.py
- turtledemo
- venv
- pwd

### Included but not working modules

The following modules can be imported, but are not functional due to the limitations of the WebAssembly VM:

- multiprocessing
- threading
- sockets

as well as any functionality that requires these.
