[pycodestyle] Stabilize behavior to ignore stub files in ambiguous-variable-name (E741)

[dylwil3]

This PR stabilizes the behavior for [ambiguous-variable-name (E741)](https://docs.astral.sh/ruff/rules/ambiguous-variable-name/#ambiguous-variable-name-e741) introduced in #13119 to ignore stub files (i.e. files ending in .pyi).

Documentation has also been updated.

There have been no open issues regarding this change for quite some time.

[github-actions]

ruff-ecosystem results

Linter (stable)

ℹ️ ecosystem check encountered linter errors. (no lint changes; 1 project error)

pandas-dev/pandas (+0 -0 violations, +0 -0 fixes)

pypa/setuptools (error)

ruff failed
  Cause: Failed to parse /home/runner/work/ruff/ruff/checkouts/pypa:setuptools/ruff.toml
  Cause: TOML parse error at line 8, column 1
  |
8 | [lint]
  | ^^^^^^
Unknown rule selector: `UP027`

Linter (preview)

ℹ️ ecosystem check encountered linter errors. (no lint changes; 1 project error)

pypa/setuptools (error)

ruff check --no-cache --exit-zero --ignore RUF9 --output-format concise --preview

ruff failed
  Cause: Failed to parse /home/runner/work/ruff/ruff/checkouts/pypa:setuptools/ruff.toml
  Cause: TOML parse error at line 8, column 1
  |
8 | [lint]
  | ^^^^^^
Unknown rule selector: `UP027`