polynomial ring over pAdics doesn't respect the sparse argument

[sagetrac-ylchapuy]

sage: R.<x> = PolynomialRing(Zp(17), sparse=True)
sage: x**(10**10)
Traceback (most recent call last)
...
MemoryError:

This should work (and be fast). The solution is probably in the file polynomial_ring_constructor.py in the function single_variate.

CC: @roed314 @xcaruso

Component: algebra

Keywords: padicIMA

Author: Julian Rüth

Reviewer: Julian Rüth

Issue created by migration from https://trac.sagemath.org/ticket/9929

[sagetrac-ylchapuy]

Description changed:

--- 
+++ 
@@ -1 +1,10 @@
-the answer is probably in the file `polynomial_ring_constructor.py` in the function `single_variate`
+
+```
+sage: R.<x> = PolynomialRing(Zp(17), sparse=True)
+sage: x**(10**10)
+Traceback (most recent call last)
+...
+MemoryError:
+```
+
+This should work (and be fast). The solution is probably in the file `polynomial_ring_constructor.py` in the function `single_variate`.

[saraedum]

Changed keywords from none to padicIMA