Add ERC721Votes for NFT-based governance (#2944)

Co-authored-by: Francisco Giordano <frangio.1@gmail.com>
Co-authored-by: Hadrien Croubois <hadrien@openzeppelin.com>

contracts/token/ERC721/extensions/draft-ERC721Votes.sol

@@ -0,0 +1,40 @@
+// SPDX-License-Identifier: MIT
+// OpenZeppelin Contracts v4.4.0 (token/ERC721/extensions/draft-ERC721Votes.sol)
+
+pragma solidity ^0.8.0;
+
+import "../ERC721.sol";
+import "../../../governance/utils/Votes.sol";
+
+/**
+ * @dev Extension of ERC721 to support voting and delegation as implemented by {Votes}, where each individual NFT counts
+ * as 1 vote unit.
+ *
+ * Tokens do not count as votes until they are delegated, because votes must be tracked which incurs an additional cost
+ * on every transfer. Token holders can either delegate to a trusted representative who will decide how to make use of
+ * the votes in governance decisions, or they can delegate to themselves to be their own representative.
+ *
+ * _Available since v4.5._
+ */
+abstract contract ERC721Votes is ERC721, Votes {
+    /**
+     * @dev Adjusts votes when tokens are transferred.
+     *
+     * Emits a {Votes-DelegateVotesChanged} event.
+     */
+    function _afterTokenTransfer(
+        address from,
+        address to,
+        uint256 tokenId
+    ) internal virtual override {
+        _transferVotingUnits(from, to, 1);
+        super._afterTokenTransfer(from, to, tokenId);
+    }
+
+    /**
+     * @dev Returns the balance of `account`.
+     */
+    function _getVotingUnits(address account) internal virtual override returns (uint256) {
+        return balanceOf(account);
+    }
+}