Document changes and release 21.2.6-1pureos3 into byzantium

debian/changelog

+mesa (21.2.6-1pureos3) byzantium; urgency=medium
+
+  * d/control: Add Breaks relation for libva-wayland2 < 2.10.0-1pureos1
+  * Remaining PureOS changes:
+    - Revert "Build with llvm-12."
+    - Update libdrm-dev dependency to match PureOS version
+    - Add python3 dependency to mesa-vulkan-drivers
+
+ -- Sebastian Krzyszkowiak <sebastian.krzyszkowiak@puri.sm>  Fri, 11 Mar 2022 17:11:54 +0100
+
 mesa (21.2.6-1pureos2) byzantium; urgency=medium
 
   * Upload to byzantium (no change rebuild)