add (overwrite with) mypy stubs, if available

stdlib/2.7/hmac.pyi

@@ -0,0 +1,11 @@
+# Stubs for hmac (Python 2)
+
+from typing import Any
+
+class HMAC:
+    def update(self, msg: str) -> None: ...
+    def digest(self) -> str: ...
+    def hexdigest(self) -> str: ...
+    def copy(self) -> HMAC: ...
+
+def new(key: str, msg: str = None, digestmod: Any = None) -> HMAC: ...