anchor.diff:    pname = "pyregion";
     version = "2.0";
   
  -  src = fetchPypi {
  -    inherit pname version;
  -    sha256 = "a8ac5f764b53ec332f6bc43f6f2193ca13e8b7d5a3fb2e20ced6b2ea42a9d094";
  +  # pypi src contains cython-produced .c files which don't compile
  +  # with python3.9
  +  src = fetchFromGitHub {
  +    owner = "astropy";
  +    repo = pname;
  +    rev = version;
  +    sha256 = "1izar7z606czcyws9s8bjbpb1xhqshpv5009rlpc92hciw7jv4kg";
     };
   
     propagatedBuildInputs = [
       pyparsing
       numpy
  -    cython
       astropy
     ];
   
  
positive.code: { lib
  , buildPythonPackage
  , fetchPypi
  , fetchpatch
  , pyparsing
  , numpy
  , cython
  , astropy
  , astropy-helpers
  , pytest
  , pytest-astropy
  }:
  
  
negative.code:     })
    ];
  
    nativeBuildInputs = [ astropy-helpers cython ];
  
    checkInputs = [ pytestCheckHook pytest-astropy ];
  
    # Disable automatic update of the astropy-helper module
    postPatch = ''
  