[1.5.x] Fix filtering during collection of paths in setup.py

for dirpath, dirnames, filenames in os.walk(django_dir):

    # Ignore PEP 3147 cache dirs and those whose names start with '.'

    for i, dirname in enumerate(dirnames):

        if dirname.startswith('.') or dirname == '__pycache__':

            del dirnames[i]

    dirnames[:] = [d for d in dirnames if not d.startswith('.') and d != '__pycache__']

    if '__init__.py' in filenames:

        packages.append('.'.join(fullsplit(dirpath)))

    elif filenames: