build(meson): use find_program('python3') (#1386)

If you really want to be sure to always find python3 when running Meson (and not some other implementation like [Muon](https://muon.build)) it is a bit better to use `find_program('python3')`, as described in https://mesonbuild.com/Reference-manual_functions.html#find_program : "if the "python3" program is requested and it is not found in the system, Meson will return its current interpreter

Co-authored-by: Jordan Bayles <bayles.jordan@gmail.com>

meson.build

@@ -73,7 +73,7 @@ if meson.is_subproject() or not get_option('tests')
   subdir_done()
 endif
 
-python = import('python').find_installation()
+python = find_program('python3')
 
 jsoncpp_test = executable(
   'jsoncpp_test', files([