opae: use python3

Also brings it closer to building again. Currently fails with:

```
CMake Error at platforms/scripts/cmake_install.cmake:54 (file):
  file INSTALL cannot find "/build/source/build/bin/afu_platform_config": No
  such file or directory.
Call Stack (most recent call first):
  platforms/cmake_install.cmake:55 (include)
  cmake_install.cmake:58 (include)

make: *** [Makefile:120: install] Error 1
```

pkgs/development/libraries/opae/default.nix

@@ -1,6 +1,6 @@
 { lib, stdenv, fetchFromGitHub, cmake
 , libuuid, json_c
-, doxygen, perl, python2, python2Packages
+, doxygen, perl, python3
 }:
 
 stdenv.mkDerivation rec {
@@ -23,10 +23,11 @@ stdenv.mkDerivation rec {
   NIX_CFLAGS_COMPILE = [
     "-Wno-error=format-truncation"
     "-Wno-error=address-of-packed-member"
+    "-Wno-array-bounds"
   ];
 
-  nativeBuildInputs = [ cmake doxygen perl python2Packages.sphinx ];
-  buildInputs = [ libuuid json_c python2 ];
+  nativeBuildInputs = [ cmake doxygen perl python3.pkgs.sphinx ];
+  buildInputs = [ libuuid json_c python3 ];
 
   # Set the Epoch to 1980; otherwise the Python wheel/zip code
   # gets very angry