build: Check for the gi python module

The introspection tests depend on the pygobject module, but we currently
are not checking if it's available at configuration time, which means we
can get build failures like:

> ModuleNotFoundError: No module named 'gi'

when running the test suite.

testsuite/introspection/meson.build

@@ -1,3 +1,4 @@
+py = import('python').find_installation('python3', modules: ['gi'])
 
 test('api',
      find_program('api.py', dirs: meson.current_source_dir()),