REGR: fix DataFrame.diff with read-only data
Closes #35559
@jreback that's what I already did (` # TODO(cython 3) update to "const diff_t[:, :] arr"`) > @jreback that's what I already did (` # TODO(cython 3) update to "const diff_t[:, :] arr"`)

ahh right, ok then. Thanks @jorisvandenbossche  @meeseeksdev backport 1.1.x
