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/meson.build

@@ -42,7 +42,7 @@ libglsl_util = static_library(
 sha1_h = custom_target(
   'git_sha1.h',
   output : 'git_sha1.h',
-  command : [prog_python2, git_sha1_gen_py, '--output', '@OUTPUT@'],
+  command : [prog_python, git_sha1_gen_py, '--output', '@OUTPUT@'],
   build_always : true, # commit sha1 can change without having touched these files
 )