Fix backward-compatibility (Spark 2.x) issue introduced with #24:

Ignore errors by calling SparkSession.getActiveSession()
(available since Spark 3.0.0) and fall-back to use the
Python logging module

sparkcc.py

@@ -159,7 +159,10 @@ def init_accumulators(self, session):
     def get_logger(self, session=None):
         """Get logger from SparkSession or (if None) from logging module"""
         if not session:
-            session = SparkSession.getActiveSession()
+            try:
+                session = SparkSession.getActiveSession()
+            except AttributeError:
+                pass # method available since Spark 3.0.0
         if session:
             return session._jvm.org.apache.log4j.LogManager \
                         .getLogger(self.name)