Documentation: Use python3 directly to check for doxylink version

When checking the version of the sphinxcontrib.doxylink module, the
Python interpreter used by Sphinx must be used, as that's where the
module will be imported from. Using the meson python_installation object
for this purpose is incorrect, as it's meant to access the Python
installation of the host machine, not the system Python interpreter on
the build machine.

For instance, when cross-compiling libcamera against a Buildroot
environment, and pointing meson in the cross-file to the build machine
Python interpreter from Buildroot, the meson python module will refer to
the latter, while Sphinx will use the former.

Fix this by using python3 directly.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>

Documentation/meson.build

@@ -124,12 +124,9 @@ if sphinx.found()
     # Many distributions do not provide a recent-enough version of the doxylink
     # module. This results in a build error with a cryptic message. Check the
     # version manually and print clear error messages.
-    py_mod = import('python')
-    py3 = py_mod.find_installation('python3')
-
     mod = 'sphinxcontrib.doxylink'
     min_version = '>=1.6.1'
-    version = run_command(py3, '-c',
+    version = run_command('python3', '-c',
                           'import @0@ ; print(@0@.__version__)'.format(mod),
                           check : false).stdout().strip()