gh-148823: Avoid importing _colorize when creating an ArgumentParser

[DavidCEllis]

This is one approach to deferring the _colorize import further.

This makes ._theme and ._decolor into properties so that _colorize is no longer imported when _set_color is called, but only when the theme is accessed.

Currently to prevent the import from subsequently being triggered when a subparser is created, this adds a 'fake' _colorless_theme which just returns empty strings for all attributes which replaces the actual colourless theme from _colorize.

Effectively this is what the colourless theme from _colorize does. I wanted to avoid duplicating all of the attribute names here or further complicating the logic, but there may be a better way to handle this.

Testing this did require an addition to the ensure_lazy_imports test helper to allow running additional code before checking for lazy imports. I think this might also be useful elsewhere when testing imports aren't being triggered earlier than intended.

Command: ./python -c 'import argparse; argparse.ArgumentParser()'

Benchmark 1: main
  Time (mean ± σ):      28.7 ms ±   3.3 ms    [User: 24.1 ms, System: 4.3 ms]
  Range (min … max):    22.2 ms …  46.4 ms    200 runs
 
Benchmark 2: lazy colorize
  Time (mean ± σ):      21.6 ms ±   5.8 ms    [User: 17.3 ms, System: 4.0 ms]
  Range (min … max):    12.1 ms …  36.0 ms    200 runs
 
Summary
  lazy colorize ran
    1.33 ± 0.39 times faster than main

Note that this uses the new lazy imports, this would need significant tweaking if it were to be backported to 3.14.

• Issue: argparse in 3.14+ is importing _colorize earlier than necessary #148823

[DavidCEllis]

Ah this pinged more people than I had anticipated.

This appears to be due to the modification to import_helper.py to allow for testing lazy imports with additional logic.