When creating a Request object scrapy applies some url encoding methods. To revert these you can utilize a custom middleware and change the url to your needs.
You could use a Downloader Middleware like this:
class MyCustomDownloaderMiddleware(object):
def process_request(self, request, spider):
request._url = request.url.replace("%5B", "[", 2)
request._url = request.url.replace("%5D", "]", 2)