Code cleanup from lgtm.com analysis

https://github.com/apache/spark/pull/23567 brought to my attention that lgtm.com has a recent analysis of Spark that turned up at least one bug: https://issues.apache.org/jira/browse/SPARK-26638

See https://lgtm.com/projects/g/apache/spark/snapshot/0655f1624ff7b73e5c8937ab9e83453a5a3a4466/files/dev/create-release/releaseutils.py?sort=name&dir=ASC&mode=heatmap#x1434915b6576fb40:1

Most of these are valid, small suggestions to clean up the code. I'm going to make a PR to implement the obvious ones.