python37: fix build on macOS

The upstream patch for distutils does not apply cleanly to Python
3.7.3's sources. (The patch applies cleanly to Python 3.7.2's sources,
but nixpkgs commit 0ddae82e6a upgraded
Python to 3.7.3.) Fix the patch to make python37 build on macOS.

pkgs/development/interpreters/python/cpython/default.nix

@@ -104,6 +104,8 @@ in with passthru; stdenv.mkDerivation {
     (
       if isPy35 then
         ./3.5/python-3.x-distutils-C++.patch
+      else if isPy37 then
+        ./3.7/python-3.x-distutils-C++.patch
       else
         fetchpatch {
           url = "https://bugs.python.org/file48016/python-3.x-distutils-C++.patch";