Fix FutureWarning by importing anndata read_* functions from anndata.io

[ren-jie-wu]

Problem

With recent versions of anndata, importing read_* functions from the top-level
anndata module triggers a FutureWarning, as these functions have been moved
to anndata.io.

This warning currently appears when importing SIMBA on environments with newer
anndata versions.

Changes

• Import read_* functions from anndata.io instead of the top-level anndata
• Keep a fallback to the old import path for compatibility with older anndata
  versions