meson: Build with Python 3

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>