Avoid deprecated Meson feature

* https://mesonbuild.com/Python-3-module.html

> This module is deprecated and replaced by the python module.
diff --git a/meson.build b/meson.build
index b2560af..aa50d8f 100644
--- a/meson.build
+++ b/meson.build
@@ -71,8 +71,7 @@
   )
 
 # tests
-#python = import('python3').find_python()
-python = find_program('python3', 'python')
+python = import('python').find_installation()
 
 jsoncpp_test = executable(
   'jsoncpp_test',