chore: run mypy as part of lint rule in Makefile (vyperlang#3771)

and remove the separate mypy rule. this makes the development workflow a
bit faster

Makefile

@@ -17,11 +17,8 @@ dev-init:
 test:
 	pytest
 
-mypy:
-	tox -e mypy
-
 lint:
-	tox -e lint
+	tox -e lint,mypy
 
 docs:
 	rm -f docs/vyper.rst