Correctly deal with modular exponentiation of polynomials when modulus is of degree <= 1

[jpflori]

sage: x = polygen(GF(5))
sage: x % 2
0

sage: x = polygen(GF(5))
sage: pow(x,100000000000,x)
Exception (FLINT memory_manager). Unable to allocate memory.

CC: @roed314 @jdemeyer @pjbruin @sagetrac-jakobkroeker

Component: algebra

Stopgaps: wrongAnswerMarker

Reviewer: Jeroen Demeyer

Issue created by migration from https://trac.sagemath.org/ticket/15866

[sagetrac-jakobkroeker]

Stopgaps: wrongAnswerMarker

[jdemeyer]

Description changed:

--- 
+++ 
@@ -1 +1,12 @@
-See #15777 comments 5, 6 and 8.
+
+```
+sage: x = polygen(GF(5))
+sage: x % 2
+0
+```
+
+```
+sage: x = polygen(GF(5))
+sage: pow(x,100000000000,x)
+Exception (FLINT memory_manager). Unable to allocate memory.
+```

[jdemeyer]

comment:5

This actually works now.

[jdemeyer]

Reviewer: Jeroen Demeyer