cleanup pyflakes warnings in sage.rings.asymptotic

[dkrenn]

There are a couple of warnings in sage.rings.asymptotic reported by pyflakes.

(This might be seen as a follow up of ​https://trac.sagemath.org/ticket/26682#comment:10)

[dkrenn]

• pyflakes warnings cleaned up. (Interestingly, the corresponding code/lines do not seem to be changed by the series of tickets ending with #26682, but the diff of the pyflakes output indicates they are not; not sure why this is so; maybe does not matter.)

• There is

  src/sage/rings/asymptotic/growth_group.py:2260: undefined name 'CartesianProductGrowthGroups'
  

  does not seem to be cleanup-able as this comes from a lazy import (so it actually is imported as should).

So, let's see what the patchbot says now ;)

---

New commits:

​8fee7c6	Trac #27759: fix pyflakes import warning
​d2deb26	Trac #27759: remove unnecessary imports

[chapoton]

recent patchbots are able to detect the lazy imports.

[chapoton]

ok