Test security warnings (#3289)

Verify that the expected warnings are emitted
with SubjectAltNameWarning emitted on all environments
due to the https server provided by httpbin_secure.

tests/__init__.py

@@ -1 +1,22 @@
 # coding: utf-8
+
+"""Requests test package initialisation."""
+
+import warnings
+
+try:
+    import urllib3 as urllib3_package
+except ImportError:
+    urllib3_package = False
+
+from requests.packages import urllib3 as urllib3_bundle
+
+if urllib3_package is urllib3_bundle:
+    from urllib3.exceptions import SNIMissingWarning
+else:
+    from requests.packages.urllib3.exceptions import SNIMissingWarning
+
+# urllib3 sets SNIMissingWarning to only go off once,
+# while this test suite requires it to always fire
+# so that it occurs during test_requests.test_https_warnings
+warnings.simplefilter('always', SNIMissingWarning)