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/broadcom/cle/meson.build

@@ -40,7 +40,7 @@ foreach _v : v3d_versions
     _name,
     input : ['gen_pack_header.py', f],
     output : _name,
-    command : [prog_python2, '@INPUT@', '@0@'.format(v)],
+    command : [prog_python, '@INPUT@', '@0@'.format(v)],
     capture : true,
   )
 endforeach
@@ -49,7 +49,7 @@ v3d_xml_h = custom_target(
   'v3d_xml.h',
   input : ['../../intel/genxml/gen_zipped_file.py', v3d_xml_files],
   output : 'v3d_xml.h',
-  command : [prog_python2, '@INPUT@'],
+  command : [prog_python, '@INPUT@'],
   capture : true,
 )