Add .DS_Store, *.swp, and .incremental_checker_cache.json to gitignore (#1950)

.DS_Store appears on Macs, *.swp is a temporary file Vim creates when
editing a file, and .incremental_checker_cache.json should probably be
ignored for the same reason .mypy_cache/ should be ignored.

Author: Michael0x2a

.gitignore

@@ -10,10 +10,15 @@ docs/build/
 /out/
 .venv/
 .mypy_cache/
+.incremental_checker_cache.json
 
 # Packages
 *.egg
 *.egg-info
 
-#IDEs
+# IDEs
 .idea
+*.swp
+
+# Operating Systems
+.DS_store