gitignore: ignore Python test artifacts (__pycache__, *.pyc, .pytest_cache)

Introduced the first pytest file in the tree
(stacks/k8s-version-upgrade/scripts/test_compat_gate.py); running it leaves an
untracked __pycache__/ dir. Ignore the standard Python build artifacts so test
runs don't show up as working-tree noise or get committed by accident.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -110,3 +110,9 @@ terraform.tfstate.backup
 # Timestamped terraform state backups (terraform.tfstate.<ts>.backup) — plaintext Tier-0
 # secrets; created by terraform state ops. The patterns above miss the timestamped form.
 terraform.tfstate.*.backup
+
+# Python test artifacts (pytest bytecode cache) — e.g. from
+# stacks/k8s-version-upgrade/scripts/test_compat_gate.py
+__pycache__/
+*.pyc
+.pytest_cache/