fix(manager upgrade): Replaced 'is' condition with isinstance
fix(manager upgrade): Replaced 'is' condition with isinstance

Beforehand, I checked if a variable 'is datetime', while the correct
fashion is using the isinstance method

scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests