Trac #20045: move wrongly imported series_precision in AsymptoticRing

`series_precision` is imported at the wrong place, which makes it appear
as a method of `AsymptoticRing`.

URL: http://trac.sagemath.org/20045
Reported by: dkrenn
Ticket author(s): Daniel Krenn
Reviewer(s): Travis Scrimshaw

src/sage/rings/asymptotic/asymptotic_ring.py

@@ -470,6 +470,7 @@
 from sage.rings.ring import Algebra
 from sage.structure.element import CommutativeAlgebraElement
 from sage.structure.unique_representation import UniqueRepresentation
+from sage.misc.defaults import series_precision
 from sage.misc.superseded import experimental
 from sage.rings.all import RIF
 
@@ -3007,7 +3008,6 @@ class AsymptoticRing(Algebra, UniqueRepresentation):
     Element = AsymptoticExpansion
 
 
-    from sage.misc.defaults import series_precision
     __default_prec__ = series_precision()  # default default-precision