PG 12 + PLPYTHON 3 compatibility

[Algunenano]

• Adds PG12 to the matrix.
• Adds some missing qualifications.
• Since _CDB_Aggregable_Attributes and CDB_ColumnNames use sql_identifier which changed from text to name, it needs to be casted in several places so that it doesn't get truncated.
• QuotaTest no longer depends on random(). Pending hiding the kb numbers since they seem to change depending on the run (after regenerating the DB, appears in all PG versions).
• OAuth tests are no longer dependent on the PG version.
• Until https://redmine.postgresql.org/issues/4868 is clarified, this changes PG12 build to depend on plpython3 instead of plpython (which defaults to plpython2). Multiple functions had to be adapted to work both with python2 and python3.

[Algunenano]

Let's make sure it works in our servers (covered by CI, staging for extra safety). At present I prefer your approach: being explicit about plpython3u.

Ups, wrong link. The correct one is https://redmine.postgresql.org/issues/4868. Right now I'm not too worried about what the decision would be, but it should be revisited when preparing the migration to PG12

[Algunenano]

I'm merging this since it doesn't involve any API changes and it shouldn't mean any changes for PG10 or PG11, but it's a good base to iterate over.