python3.pkgs.tinycss: fix build

The build was broken by the python 3.7 switch, which caused an
incompatible change in the way cython generates files:

Kozea/tinycss#17

This is solved by removing the pre-generated file and re-generating it
at build time.

pkgs/development/python-modules/tinycss/default.nix

@@ -3,6 +3,7 @@
 , fetchPypi
 , pytest
 , python
+, cython
 , cssutils
 , isPyPy
 }:
@@ -18,6 +19,17 @@ buildPythonPackage rec {
 
   checkInputs = [ pytest ];
   propagatedBuildInputs = [ cssutils ];
+  nativeBuildInputs = [
+    cython
+  ];
+
+  preBuild = ''
+    # Force cython to re-generate this file. If it is present, cython will
+    # think it is "up to date" even though it was generated with an older,
+    # incompatible version of cython. See
+    # https://github.com/Kozea/tinycss/issues/17.
+    rm tinycss/speedups.c
+  '';
 
   checkPhase = ''
     py.test $out/${python.sitePackages}