gh-144384: Lazily import _colorize

[hugovk]

Lazily import _colorize to avoid slow construction/import when not needed.

The easily testable:

Before	After
difflib: 10ms	2ms (80% faster)
doctest: 48ms	40ms (17% faster)
json.tool: 13ms	6ms (54% faster)
pdb: 43ms	37ms (14% faster)
traceback: 15ms	8ms (47% faster)
unittest: 20ms	14ms (30% faster)

Some indirect improvements:

Before	After
asyncio: 47ms	27ms (43% faster)
logging: 17ms	10ms (41% faster)

traceback needs some extra handling: add a shutdown theme, so if it attempts to reify _colorize during shutdown when the import machinery is no longer around, it has a no-op fallback.

Also add ensure_lazy_imports tests.

• Issue: _colorize module is slow to import, affecting traceback and logging modules #144384

[picnixz]

Can we have a convention for lazy imports

[hugovk]

Can we have a convention for lazy imports

Yes, given we don't have a convention, and the SC left it the details to linters and auto-formatters, let's generally follow the Ruff/isort convention:

import json
import os
import subprocess
from collections import defaultdict
from pathlib import Path
from typing import Final
lazy import ast
lazy import shutil
lazy from dataclasses import dataclass

• astral-sh/ruff#23762
• astral-sh/ruff#21305
• PyCQA/isort#2503
• PyCQA/isort#2462

[StanFromIreland]

Seeing #149321, I think this should wait till that is somewhat figured out.