replace 512 bit dh key with a 2014 bit one (closes #23844)

Patch by Cédric Krier.
diff --git a/Lib/test/test_ssl.py b/Lib/test/test_ssl.py
index 97e5654..e58f55a 100644
--- a/Lib/test/test_ssl.py
+++ b/Lib/test/test_ssl.py
@@ -66,7 +66,7 @@
 NOKIACERT = data_file("nokia.pem")
 NULLBYTECERT = data_file("nullbytecert.pem")
 
-DHFILE = data_file("dh512.pem")
+DHFILE = data_file("dh1024.pem")
 BYTES_DHFILE = DHFILE.encode(sys.getfilesystemencoding())