meson: Build with Python 3

Now that all the build scripts are compatible with both Python 2 and 3,
we can flip the switch and tell Meson to use the latter.

Since Meson already depends on Python 3 anyway, this means we don't need
two different Python stacks to build Mesa.

Signed-off-by: Mathieu Bridon <bochecha@daitauha.fr>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>

src/mapi/shared-glapi/meson.build

@@ -31,7 +31,7 @@ shared_glapi_mapi_tmp_h = custom_target(
   'shared_glapi_mapi_tmp.h',
   input : [mapi_abi_py, gl_and_es_api_files],
   output : 'glapi_mapi_tmp.h',
-  command : [prog_python2, '@INPUT0@', '--printer', 'shared-glapi', '@INPUT1@'],
+  command : [prog_python, '@INPUT0@', '--printer', 'shared-glapi', '@INPUT1@'],
   depend_files : api_xml_files,
   capture : true,
 )