Don't use requests.packages.urllib3

This was a really old hack around forks in urllib3 in early versions of
requests 2. With newer requests we don't need this any more.

requests_mock/response.py

@@ -18,8 +18,8 @@
 from requests.cookies import MockRequest, MockResponse
 from requests.cookies import RequestsCookieJar
 from requests.cookies import merge_cookies, cookiejar_from_dict
-from requests.packages.urllib3.response import HTTPResponse
 from requests.utils import get_encoding_from_headers
+from urllib3.response import HTTPResponse
 
 from requests_mock import exceptions