support separation of Numeric/Float

support separation of Numeric/Float

in [1] we are considering separating Numeric and Float.
For Alembic PostgreSQL backend we need this isinstance therefore
to check for both Numeric and Float.
By keeping it to these two types, rather than targeting the
NumericCommon type being added in [1], the patch can work with
SQLAlchemy without the separation change as well.

[1] https://gerrit.sqlalchemy.org/c/sqlalchemy/sqlalchemy/+/3587

Change-Id: I0c956ba8797e38a62ea630ab65cd53779bbf1972