pep8
- enhance only_on() to work with compound specs
- fix "temporary_tables" requirement
- SQL Server 2012 now recommends VARCHAR(max), NVARCHAR(max),
VARBINARY(max) for large text/binary types.  The MSSQL dialect will
now respect this based on version detection, as well as the new
``deprecate_large_types`` flag.
fixes #3039