From the cx_Oracle installation instructions:
Oracle client libraries versions 12.2, 12.1 and 11.2 are supported
From the cx_Oracle installation instructions on interoperability:
Oracle’s standard client-server network interoperability allows
connections between different versions of Oracle Client libraries and
Oracle Database. For certified configurations see Oracle Support’s Doc
ID 207303.1. In summary, Oracle Client 12.2 can connect to Oracle
Database 11.2 or greater. Oracle Client 12.1 can connect to Oracle
Database 10.2 or greater. Oracle Client 11.2 can connect to Oracle
Database 9.2 or greater. The technical restrictions on creating
connections may be more flexible. For example Oracle Client 12.2 can
successfully connect to Oracle Database 10.2.
Your quick solution is to install the 12.1 or 12.2 Instant Client RPMs. (The longer solution is to upgrade your DB to a supported release!)