Scala 2.13.14 test:

scala-cli -j system -S 2.13.14 --dep org.python:jython-standalone:2.7.1b3

scala> val x = new org.python.core.PyList
val x: org.python.core.PyList = []

scala> import scala.jdk.CollectionConverters._
import scala.jdk.CollectionConverters._

scala> x.asScala
val res0: scala.collection.mutable.Buffer[_] = Buffer()

same simple test with Scala 3.3.3:

 scala-cli -j system -S 3.3.3 --dep org.python:jython-standalone:2.7.1b3
Welcome to Scala 3.3.3 (21.0.3, Java OpenJDK 64-Bit Server VM).
Type in expressions for evaluation. Or try :help.
                                                                                                                                          
scala> val x = new org.python.core.PyList
val x: org.python.core.PyList = []
                                                                                                                                          
scala> import scala.jdk.CollectionConverters.*
                                                                                                                                          
scala> x.asScala
-- [E008] Not Found Error: -----------------------------------------------------
1 |x.asScala
  |^^^^^^^^^
  |value asScala is not a member of org.python.core.PyList
1 error found

Take into account that PyList is just a wrapper around java.util.List:

public class PyList extends PySequenceList implements List {
  ....
}