get_columns fails for certain column types

[kadler]

base.py:1128: SAWarning: Did not recognize type 'TIMESTMP' of column 'CREATEDTIMESTAMP'
util.warn("Did not recognize type '%s' of column '%s'" %

This is because the keys in ischema_names don't match the values returned by QSYS2.SYSCOLS, eg. it has "TIMESTAMP" instead of "TIMESTMP".

Because sqlalchemy-ibmi is based on ibm_db_sa which supports multiple Db2 flavors, I'm not sure if this bug was due to differences in SYSCOLS between LUW and IBM i or if it was just always wrong.

I've got some code for this, in addition to refactoring how get_columns is implemented.

[kadler]

Code is on the export-types branch: https://github.com/IBM/sqlalchemy-ibmi/commits/export-types/

Specifically this commit: f6d2685