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/vulkan/util/meson.build

@@ -31,7 +31,7 @@ vk_enum_to_str = custom_target(
   input : ['gen_enum_to_str.py', vk_api_xml[0]],
   output : ['vk_enum_to_str.c', 'vk_enum_to_str.h'],
   command : [
-    prog_python2, '@INPUT0@', '--xml', '@INPUT1@', '--outdir',
+    prog_python, '@INPUT0@', '--xml', '@INPUT1@', '--outdir',
     meson.current_build_dir()
   ],
 )