Backport of 7fd1b35e from master

db_parameters should respect an already existing db_type method and
return that as its type string. In particular, this was causing some
fields from gis to not be generated.

Thanks to @bigsassy and @blueyed for their work on the patch.

Also fixed #22260

Thanks Piotr Kasprzyk for help with the patch.

Signed-off-by: Jason Myers <jason@jasonamyers.com>

The SubfieldBase's descriptor caused an AttributeError when accessed
from the class. Introspection didn't like that.

Patch by Trac alias supervacuo.

Should be unneeded with Python 2.7 and up.
Added some unicode_literals along the way.

Gives much nicer errors when it fails.