build: fix naming of Python 2 loader

The Python 2 loader is still named `pythonloader`. This fixes the name
of the loader so that it is properly loaded.

loaders/python/meson.build

-python2_loader_name = 'python2loader'
+python2_loader_name = 'pythonloader'
 
 pygobject_version = pygobject_dep.version()
 pygobject_version_arr = pygobject_version.split('.')