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- [x] Add new / update outdated documentation

from pytest_pyodide import run_in_pyodide


@run_in_pyodide(packages=["cffi"])
def test_cffi_asprintf(selenium):
    from cffi import FFI

    ffi = FFI()
    ffi.cdef(
        """int asprintf(char** buf, const char *format, ...);   // copy-pasted from the man page"""
    )
    C = ffi.dlopen(None)  # loads the entire C namespace
    buf = ffi.new("char**")
    arg1 = ffi.new("char[]", b"wo")
    arg2 = ffi.new("char[]", b"ld")
    C.asprintf(buf, b"hello %sr%s", arg1, arg2)
    assert ffi.string(buf[0]).decode() == "hello world"

from pytest_pyodide import run_in_pyodide


@run_in_pyodide(packages=["cffi"])
def test_cffi_asprintf(selenium):
    from cffi import FFI

    ffi = FFI()
    ffi.cdef(
        """int asprintf(char** buf, const char *format, ...);   // copy-pasted from the man page"""
    )
    C = ffi.dlopen(None)  # loads the entire C namespace
    buf = ffi.new("char**")
    arg1 = ffi.new("char[]", b"wo")
    arg2 = ffi.new("char[]", b"ld")
    C.asprintf(buf, b"hello %sr%s", arg1, arg2)
    assert ffi.string(buf[0]).decode() == "hello world"
