update CMakelist to incorporate the Python3 CTemplate patch

[benczaja]

Hi Guys,

I see that you have a nice patch for CTemplate/Python3 for the "CPU" version of HemePure. It just simply looks like the same functionality hasn't been migrated to this repo.

Can you also incorporate the Python3 patch for CTemplate in this repo as well.
I think this is useful information for you guys...

https://github.com/UCL-CCS/HemePure/blob/c51d711fb4ede9cc4a54306a13990c5230156dcd/dep/CMakeLists.txt#L201
and
https://github.com/UCL-CCS/HemePure/blob/c51d711fb4ede9cc4a54306a13990c5230156dcd/dep/CMakeLists.txt#L227

And
https://github.com/UCL-CCS/HemePure/blob/master/dep/patches/ctemplate_python3.diff

I don't think that this should be hard to change...
I can open up a PR if that is preferred.
Best,
Ben

[benczaja]

See PR #9