[build] Remove .mypy_cache with clean.all Makefile target

Makefile

@@ -67,4 +67,9 @@ clean:
 	rm -rf dist *.egg-info
 
 clean.all: clean
-	find . -type d -name __pycache__ -o -name .pytest_cache -exec rm -r "{}" +
+	find .  \
+		-type d \
+		-name __pycache__ \
+		-o -name .pytest_cache \
+		-o -name .mypy_cache \
+		-exec rm -r "{}" +