python3: Fix 'sysconfig_path' for platform-dependant paths

Include 'platbase' for stripping the prefix for 'platlib' and 'platinclude'.

This is necessary for installing platform-dependant Python modules such as GI overrides.

mesonbuild/modules/python3.py

@@ -68,7 +68,7 @@ class Python3Module(ExtensionModule):
             raise mesonlib.MesonException('{} is not a valid path name {}.'.format(path_name, valid_names))
 
         # Get a relative path without a prefix, e.g. lib/python3.6/site-packages
-        path = sysconfig.get_path(path_name, vars={'base': ''})[1:]
+        path = sysconfig.get_path(path_name, vars={'base': '', 'platbase': ''})[1:]
         return ModuleReturnValue(path, [])