Fix Python API calling into Scala code

[EnricoMi]

Running the tests via spark-submit after installing the built package uncovered some issues with the Python API.

Fixes issues like

py4j.protocol.Py4JJavaError: An error occurred while calling z:java.lang.Class.forName.
: java.lang.ClassNotFoundException: uk.co.gresearch.spark.UnpersistHandle$
	at java.net.URLClassLoader.findClass(URLClassLoader.java:387)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:419)
	at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:352)
	at java.lang.ClassLoader.loadClass(ClassLoader.java:352)
	at java.lang.Class.forName0(Native Method)
	at java.lang.Class.forName(Class.java:264)
	at sun.reflect.GeneratedMethodAccessor4.invoke(Unknown Source)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at py4j.reflection.MethodInvoker.invoke(MethodInvoker.java:244)
	at py4j.reflection.ReflectionEngine.invoke(ReflectionEngine.java:357)
	at py4j.Gateway.invoke(Gateway.java:282)
	at py4j.commands.AbstractCommand.invokeMethod(AbstractCommand.java:132)
	at py4j.commands.CallCommand.execute(CallCommand.java:79)
	at py4j.GatewayConnection.run(GatewayConnection.java:238)
	at java.lang.Thread.run(Thread.java:750)

[github-actions]

Test Results

     364 files  +  42       364 suites  +42   1h 22m 2s ⏱️ + 22m 44s
     441 tests +  38       441 ✔️ +  38  0 💤 ±0  0 ❌ ±0 
11 156 runs  +532  11 156 ✔️ +532  0 💤 ±0  0 ❌ ±0 

Results for commit 6faf24a. ± Comparison against base commit 02568b8.

♻️ This comment has been updated with latest results.