[python-pbr/f19] (3 commits) ...remove requirements.txt, require python-pip

[Pádraig Brady]

Summary of changes:

  f13711f... update to 0.5.21 (rhbz#990008) (*)
  318a0aa... pbr now requires pip (*)
  cb36574... remove requirements.txt, require python-pip (*)

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