Fix function state mutability warning (#2327)

Changes state mutability of granularity function from view to pure.

contracts/token/ERC777/ERC777.sol

@@ -113,7 +113,7 @@ contract ERC777 is Context, IERC777, IERC20 {
      *
      * This implementation always returns `1`.
      */
-    function granularity() public view override returns (uint256) {
+    function granularity() public pure override returns (uint256) {
         return 1;
     }