title: Constrain upstream dependencies in pip
body: <p>I need to specify a version for a package that one of my dependencies depends on, but my package does not directly depend on. Say my CI is trying to build package_a, it depends on package_b and does not depend on package_c. But package_b depends on package_c and it needs to be a particular version. In other words, "don't install this, but if you end up installing this, please install this version."</p>
  
  <p>This appears to be doable in conda with <a href="https://github.com/conda/conda-build/pull/2001" rel="nofollow noreferrer">run_constrainted</a> but I cannot determine if it is possible to do in pip in the requirements.</p>
  
  <p>A less appealing workaround would just be to add this other package (package_c) as another requirement.</p>