Update dependencies

Update to latest version of beat/beat.backend.python> which fixes an issue with multiprocessing handling.

dev.yml

@@ -12,7 +12,7 @@ dependencies:
 
   # beat dependencies matching release.cfg
   - beat.core=1.10.3
-  - beat.backend.python=1.7.6
+  - beat.backend.python=1.7.7
   - beat.cmdline=1.7.1
 
   # requirements.txt, they are indirectly pinned through the above