Add a changelog entry

(cherry picked from commit 4e60480fdc5fffaa1cd70d1e5bdade81ed1b4e14)

CHANGELOG.md

 # Changelog
 
+## Unreleased
+
+ * `ERC721`: made `_approve` an internal function (was private).
+
 ## 3.4.0 (2021-02-02)
 
  * `BeaconProxy`: added new kind of proxy that allows simultaneous atomic upgrades. ([#2411](https://github.com/OpenZeppelin/openzeppelin-contracts/pull/2411))