Warn when slicing twice

[pquentin]

This is an alternative to merging #1771 directly.

Given the following file:

from elasticsearch_dsl import Search

s = Search()
s[10:20][2:]

Running it gives:

testwarn.py:4: DeprecationWarning: Slicing multiple times currently has no effect but will be supported in a future release. See https://github.com/elastic/elasticsearch-dsl-py/pull/1771 for more details
  s[10:20][2:]

Note that this is only displayed because the warning comes from the executed module. Users will only see it if they use pytest or use -Wall.