[Maybe?] potential issue on MacOS if python is not installed in home-brew

[tdavidcl]

I got some weird linking errors at the end of the compilation when python was not installed

ld: library 'python3.9' not found
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
[5/7] Linking CXX executable shamrock_test
ld: library 'python3.9' not found
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
[6/7] Linking CXX shared module shamrock.cpython-39-darwin.so
ninja: build stopped: subcommand failed.

Here is the pybind11 section of the Cmake configure

   ---- Pybind11 section ----
CMake Warning at cmake/ShamConfigurePybind11.cmake:17 (message):
  PYTHON_EXECUTABLE is not defined, please set it as follows:

  -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
Call Stack (most recent call first):
  CMakeLists.txt:82 (include)


-- Trying to autodetect python executable: python3
-- PYTHON_EXECUTABLE : /Library/Developer/CommandLineTools/usr/bin/python3
-- Using git submodule pybind11
CMake Deprecation Warning at external/pybind11/CMakeLists.txt:13 (cmake_minimum_required):
  Compatibility with CMake < 3.10 will be removed from a future version of
  CMake.

  Update the VERSION argument <min> value.  Or, use the <min>...<max> syntax
  to tell CMake that the project requires at least <min> but has been updated
  to work with policies introduced by <max> or earlier.


-- pybind11 v2.13.6 
-- Found PythonInterp: /Library/Developer/CommandLineTools/usr/bin/python3 (found suitable version "3.9.6", minimum required is "3.7")
-- Found PythonLibs: python3.9

A easy solution is to enforce python in the list of brew packages