Stop tracking .DS_Store; add to .gitignore

macOS Finder regenerates .DS_Store on every directory visit, producing
spurious 'M .DS_Store' noise in git status. Untrack the existing one
and add the pattern to .gitignore so it stays out of future diffs.

Author: dcj

.DS_Store

@@ -8,3 +8,4 @@ build/
 .venv/
 *.so
 .pytest_cache/
+.DS_Store