Use CMake features to find NumPy - #333

I know it has taken me a bit to figure out the exact linking needed for the tests, but now, the current test failures seem to be due to the test environment not having the libpython3.so available:

CMake Error at /home/runner/micromamba/envs/xtensor-python/share/cmake-4.4/Modules/FindPackageHandleStandardArgs.cmake:290 (message):
  Could NOT find Python3 (missing: Python3_LIBRARIES Development.Embed)
  (found version "3.15.0")

Looking at this link, it appears that starting with Python 3.15, the python package in conda no longer provides libpython3.so, and you must add the libpython package as well. The error message seems to support this, as it found a 3.15 interpreter, but not the dynamic library. I don't think any tests have been run on this repo since python 3.15 came out ? Because I think if the pipeline on master were to be re-run, you would find that tests are broken due to this issue. To fix this, I am going to just specify python and libpython as dependencies in environment-dev.yml to try and fix the problem.

I don't think any tests have been run on this repo since python 3.15 came out ?

I don't think so.

Also do you want to open another PR with a very small change that I can merge, so that you don't need to wait for approval to run the CI in this one?

Yes, that is a good idea. In any case, it was clear that python and libpython need to be added to the test environment for the build to work now. I will put those in a separate PR while trying to fix the remaining issues with this one.