opae: use python3
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
```

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