Coerce float Python type to Float; ensure Python float coming back

Coerce float Python type to Float; ensure Python float coming back

Added some extra strictness to the handling of Python "float" values
passed to SQL statements.  A "float" value will be associated with the
:class:`.Float` datatype and not the Decimal-coercing :class:`.Numeric`
datatype as was the case before, eliminating a confusing warning
emitted on SQLite as well as unecessary coercion to Decimal.

Change-Id: I1bb1810ff1d198c0d929ccba5656e55401d74119
Fixes: #4017