docs(types) Fix missing import from sqlalchemy (#7978)

docs(types) Fix missing import from sqlalchemy (#7978)

* docs(types) Fix missing import from sqlalchemy

The sample code is missing the import of Enum from sqlalchemy, which might confuse the reader, since we are using another enum type from Python itself here. So it makes sense to clarify that here.

* fix whitespaces

Change-Id: I019bbed8a7278f60e7239160ea4c99ecd2519d3b

Co-authored-by: Federico Caselli <cfederico87@gmail.com>
(cherry picked from commit f89a202d60215b13e3733a3ea950473962b3cf67)