Work around Debian's urllib3 security patch

It defaults cert_reqs to 'CERT_REQUIRED', where upstream urllib3 would defer to
ssl_context, when provided.

Bug-Debian: https://bugs.debian.org/1054212
--- a/tests/test_sslcontext.py
+++ b/tests/test_sslcontext.py
@@ -49,7 +49,7 @@
     assert ctx.check_hostname is False
     assert ctx.verify_mode == ssl.CERT_NONE
 
-    with urllib3.PoolManager(ssl_context=ctx) as http, pytest.warns(
+    with urllib3.PoolManager(ssl_context=ctx, cert_reqs='CERT_NONE') as http, pytest.warns(
         InsecureRequestWarning
     ) as w:
         http.request("GET", "https://expired.badssl.com/")
