[sagemath/f20] (6 commits) ...Update to sagemath 5.12

[pcpa]

Summary of changes:

  11b0fd5... Update to work with latest rawhide atlas (*)
  fb141f7... Remove incorrect revert of previous bz fixes (*)
  550801c... Add missing zn_poly-devel build requires (*)
  e3260dd... Add python-sphinx to build requires (*)
  4f797f2... Correct release downgrade and changelog truncation (*)
  d4a07df... Update to sagemath 5.12 (*)

(*) This commit already existed in another branch; no separate mail sent