ci: migrate from flake8 to ruff (#2120)

[cfxegbert]

See #2120

New files

.github/workflows/ruff.yaml

• Runs ruff check on PR/push when Python sources or lint config change
• Skips vendor/, data/, and _version.py
• Uses pinned actions/checkout and astral-sh/ruff-action

ruff.toml

• Python 3.8 target, 120-char lines
• Excludes src/rez/vendor and src/rez/data
• Enables rule sets that mirror flake8 (pycodestyle E/W, Pyflakes F, DTZ, ISC, etc.)
• Global ignores:
  • E722— bare except
  • E741— ambiguous variable names

Per-file ignores for known existing violations (no code changes in this branch):

File	Rule	Reason
src/rezgui/widgets/TimestampWidget.py	E731	Lambda assigned to variable (fn = lambda ...)
src/rez/package_order.py	E721	Type comparisons
src/rezgui/objects/Config.py	E721	Type comparisons
install.py	E402, F401	Late imports / unused import
setup.py	E402	Late imports
release-rez.py	E402, DTZ011	Late imports / date.today()
src/rez/tests/*.py	F821	Undefined names (rex builtins)
src/rez/bind/_pymodule.py	F821	Undefined names (bind env)
src/rez/utils/filesystem.py	F821	Undefined names
src/rez/resolver.py	F821	Undefined names
src/rez/cli/release.py	F821	Undefined names
src/rezplugins/release_vcs/git.py	F821	Undefined names
src/rez/cli/_main.py	ISC002	Multi-line string concatenation
src/rez/tests/test_formatter.py	E742	Ambiguous class name I
src/rezgui/widgets/ContextToolsWidget.py	F401	Unused import
src/support/package_utils/set_authors.py	INP001	Implicit namespace package

Out of scope (this branch)

• No application logic changes
• No whitespace or comment refactors

Summary

Infrastructure-only slice of the flake8 → Ruff migration: add Ruff config and CI with per-file exemptions so lint passes on the existing codebase without touching source files yet.

I used some help from an LLM to create the summary for this PR

[JeanChristopheMorinPerso]

Hi @cfxegbert, is there parity with flake8 rules? If so, I think it would be safe to completely replace flake8 with ruff and delete the flake8 job from the CI.