implement asec/acsc series at infinity

[rwst]

The known series for asec/acsc are not implemented:

sage: asec(x).series(x==oo)
(arcsec(+Infinity)) + Order(+Infinity)
sage: acsc(x).series(x==oo)
(arccsc(+Infinity)) + Order(+Infinity)

• http://mathworld.wolfram.com/InverseSecant.html
• http://mathworld.wolfram.com/InverseCosecant.html

This would need series growing in 1/x so is not possible at the moment.

Component: calculus

Issue created by migration from https://trac.sagemath.org/ticket/20090

[rwst]

Description changed:

--- 
+++ 
@@ -10,4 +10,4 @@
 * http://mathworld.wolfram.com/InverseSecant.html
 * http://mathworld.wolfram.com/InverseCosecant.html
 
-This would be possibly in a next Pynac version. If so, this ticket should doctest that.
+This would need series growing in `1/x` so is not possible at the moment.