add more minor 3.7 new features (#2000)

part of #1965

stdlib/2and3/hmac.pyi

@@ -35,3 +35,6 @@ def compare_digest(a: str, b: str) -> bool: ...
 def compare_digest(a: bytes, b: bytes) -> bool: ...
 @overload
 def compare_digest(a: bytearray, b: bytearray) -> bool: ...
+
+if sys.version_info >= (3, 7):
+    def digest(key: _B, msg: _B, digest: str) -> bytes: ...