bpo-19180: Updated references for RFC 1750, RFC 3280 & RFC 4366 (GH-148)

* RFC 1750 has been been obsoleted by RFC 4086.
* RFC 3280 has been obsoleted by RFC 5280.
* RFC 4366 has been obsoleted by RFC 6066.
diff --git a/Modules/_ssl.c b/Modules/_ssl.c
index d318b25..9077d0a 100644
--- a/Modules/_ssl.c
+++ b/Modules/_ssl.c
@@ -4440,7 +4440,7 @@
 Mix string into the OpenSSL PRNG state.
 
 entropy (a float) is a lower bound on the entropy contained in
-string.  See RFC 1750.
+string.  See RFC 4086.
 [clinic start generated code]*/
 
 static PyObject *