Read allowance from overridable function in increase/decrease (#3213)

contracts/token/ERC20/ERC20.sol

@@ -180,7 +180,7 @@ contract ERC20 is Context, IERC20, IERC20Metadata {
      */
     function increaseAllowance(address spender, uint256 addedValue) public virtual returns (bool) {
         address owner = _msgSender();
-        _approve(owner, spender, _allowances[owner][spender] + addedValue);
+        _approve(owner, spender, allowance(owner, spender) + addedValue);
         return true;
     }
 
@@ -200,7 +200,7 @@ contract ERC20 is Context, IERC20, IERC20Metadata {
      */
     function decreaseAllowance(address spender, uint256 subtractedValue) public virtual returns (bool) {
         address owner = _msgSender();
-        uint256 currentAllowance = _allowances[owner][spender];
+        uint256 currentAllowance = allowance(owner, spender);
         require(currentAllowance >= subtractedValue, "ERC20: decreased allowance below zero");
         unchecked {
             _approve(owner, spender, currentAllowance - subtractedValue);