docs: fix 'falsey' -> 'falsy' and modernize stale docs.python.org URLs

[MukundaKatta]

Summary

Partial fix for #1182 (the wording typo + stale docs.python.org URLs; the Filtering a list `while i in a` example is intentionally not touched — it's a separate rewrite the maintainer can steer).

• `docs/writing/style.rst`: `'attr is falsey!'` -> `'attr is falsy!'` (the Python community / PEP / stdlib wording is "falsy").
• Upgrade unversioned `http://docs.python.org/...\` links (and one `docs.python.org/2/` argparse link) to `https://docs.python.org/3/...\`:
  • `style.rst` — Truth Value Testing, list comprehensions, generator expressions
  • `structure.rst` — packages tutorial
  • `cli.rst` — argparse (was explicitly pinned to Python 2)
  • `learning.rst` — Tutorial and Language Reference

Refs #1182

Testing

Docs-only change. 8 lines touched across 4 files, line endings preserved.