Make contracts abstract if they had internal constructors (#2383)

contracts/access/Ownable.sol

@@ -15,7 +15,7 @@ import "../GSN/Context.sol";
  * `onlyOwner`, which can be applied to your functions to restrict their use to
  * the owner.
  */
-contract Ownable is Context {
+abstract contract Ownable is Context {
     address private _owner;
 
     event OwnershipTransferred(address indexed previousOwner, address indexed newOwner);

contracts/introspection/ERC165.sol

@@ -10,7 +10,7 @@ import "./IERC165.sol";
  * Contracts may inherit from this and call {_registerInterface} to declare
  * their support of an interface.
  */
-contract ERC165 is IERC165 {
+abstract contract ERC165 is IERC165 {
     /*
      * bytes4(keccak256('supportsInterface(bytes4)')) == 0x01ffc9a7
      */

contracts/payment/PullPayment.sol

@@ -22,7 +22,7 @@ import "./escrow/Escrow.sol";
  * instead of Solidity's `transfer` function. Payees can query their due
  * payments with {payments}, and retrieve them with {withdrawPayments}.
  */
-contract PullPayment {
+abstract contract PullPayment {
     Escrow private _escrow;
 
     constructor () {

contracts/utils/Pausable.sol

@@ -13,7 +13,7 @@ import "../GSN/Context.sol";
  * the functions of your contract. Note that they will not be pausable by
  * simply including this module, only once the modifiers are put in place.
  */
-contract Pausable is Context {
+abstract contract Pausable is Context {
     /**
      * @dev Emitted when the pause is triggered by `account`.
      */

contracts/utils/ReentrancyGuard.sol

@@ -18,7 +18,7 @@ pragma solidity ^0.7.0;
  * to protect against it, check out our blog post
  * https://blog.openzeppelin.com/reentrancy-after-istanbul/[Reentrancy After Istanbul].
  */
-contract ReentrancyGuard {
+abstract contract ReentrancyGuard {
     // Booleans are more expensive than uint256 or any type that takes up a full
     // word because each write operation emits an extra SLOAD to first read the
     // slot's contents, replace the bits taken up by the boolean, and then write