src/sage/rings/power_series_ring_element.pyx: Add import in doctest
References
#36307 - `sage.rings`: Remove code deprecated in #23204, #24483, #24371, #24511, #25848, #26105, #28481, #29010, #29412, #30332, #30372, #31345, #32375, #32606, #32610, #32612, #32641, #32660, #32750, #32869, #33602