Fix Cython vtable warnings in padic_generic_element.pyx

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Taken as part from #24609

CC:  @saraedum

Component: **cython**

Author: **Julian Rüth**

Branch/Commit: **[`2c75526`](https://github.com/sagemath/sagetrac-mirror/commit/2c75526a8733b503d6f3c9924cc9a7b760754b2f)**

Reviewer: **Jeroen Demeyer**

_Issue created by migration from https://trac.sagemath.org/ticket/27417_

