python: install_sources() should default to pure, following the doc

As stated by the doc, default to install python sources to purelib
location, as they should not depend on platform.

This also fixes discrepancy between get_install_dir() and
install_sources() locations.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

mesonbuild/modules/python.py

@@ -361,7 +361,7 @@ class PythonInstallation(ExternalProgramHolder):
 
     @permittedKwargs(['pure', 'subdir'])
     def install_sources_method(self, args, kwargs):
-        pure = kwargs.pop('pure', False)
+        pure = kwargs.pop('pure', True)
         if not isinstance(pure, bool):
             raise InvalidArguments('"pure" argument must be a boolean.')