Update optional DB dependency from obsolete 'cx_Oracle' to 'oracledb'

[cjbj]

The python-oracledb driver is the upgrade from cx_Oracle, see the release announcement. This was first released several years ago and it's past time for oci-cli to update, e.g. to update setup.py and change:

extras = {
    'db': ['cx_Oracle==8.3']
}

to:

extras = {
    'db': ['oracledb']
}

This new version of the driver for Oracle Database connectivity has various advantages, notably it doesn't require Oracle Client libraries to be installed.