Initial ORM typing layout

introduces:

1. new mapped_column() helper
2. DeclarativeBase helper
3. declared_attr has been re-typed
4. rework of Mapped[] to return InstrumentedAtribute for
   class get, so works without Mapped itself having expression
   methods
5. ORM constructs now generic on [_T]

also includes some early typing work, most of which will
be in later commits:

1. URL and History become typing.NamedTuple
2. come up with type-checking friendly way of type
   checking cy extensions, where type checking will be applied
   to the py versions, just needed to come up with a succinct
   conditional pattern for the imports

References: #6810
References: #7535
References: #7562
Change-Id: Ie5d9a44631626c021d130ca4ce395aba623c71fb

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