[fix] use poolrequests in ddg engines

searx/engines/duckduckgo.py

 """
 
 from lxml.html import fromstring
-from requests import get
 from json import loads
 from searx.engines.xpath import extract_text
+from searx.poolrequests import get
 from searx.url_utils import urlencode
 
 # engine dependent config

searx/engines/duckduckgo_images.py

  @todo        avoid extra request
 """
 
-from requests import get
 from json import loads
 from searx.engines.xpath import extract_text
 from searx.engines.duckduckgo import _fetch_supported_languages, supported_languages_url, get_region_code
+from searx.poolrequests import get
 from searx.url_utils import urlencode
 
 # engine dependent config