rename Z3 to z3 to conform with snakecase

Author: christopher-wild

causal_testing/specification/variable.py

@@ -14,10 +14,10 @@
 
 # Declare type variable
 T = TypeVar("T")
-Z3 = TypeVar("Z3")
+z3 = TypeVar("Z3")
 
 
-def z3_types(datatype: T) -> Z3:
+def z3_types(datatype: T) -> z3:
     """Cast datatype to Z3 datatype
     :param datatype: python datatype to be cast
     :return: Type name compatible with Z3 library