Thank you for your answer. We tried it. It worked using spark-submit on the command line, but it did not work in Zeppelin (in which we configured the config flags for Spark).
What we then tried, was setting the Spark submit options in Ambari (with no luck):
SPARK_SUBMIT_OPTIONS="--conf spark.pyspark.virtualenv.enabled=true --conf
spark.pyspark.virtualenv.python_version=3.6 --conf spark.pyspark.virtualenv.requirements=/home/zeppelin/requirements.txt"