add (overwrite with) mypy stubs, if available

stdlib/2.7/md5.pyi

@@ -0,0 +1,11 @@
+# Stubs for Python 2.7 md5 stdlib module
+
+class md5(object):
+    def update(self, arg: str) -> None: ...
+    def digest(self) -> str: ...
+    def hexdigest(self) -> str: ...
+    def copy(self) -> md5: ...
+
+def new(string: str = None) -> md5: ...
+blocksize = 0
+digest_size = 0