point to unmerged AbstractPPL branch

Author: penelopeysm

test/Project.toml

@@ -50,3 +50,6 @@ ReverseDiff = "1"
 StableRNGs = "1"
 Zygote = "0.6, 0.7"
 julia = "1.10"
+
+[sources]
+AbstractPPL = {url = "https://github.com/TuringLang/AbstractPPL.jl", rev = "py/hasgetvalue"}