Fixed #631 -- deprecate all of OpenSSL.rand (#658)

* Fixed #631 -- deprecate all of OpenSSL.rand

* syntax fix

* flake8
diff --git a/src/OpenSSL/rand.py b/src/OpenSSL/rand.py
index cf2053f..e2f6373 100644
--- a/src/OpenSSL/rand.py
+++ b/src/OpenSSL/rand.py
@@ -2,6 +2,7 @@
 PRNG management routines, thin wrappers.
 """
 
+import warnings
 from functools import partial
 
 from six import integer_types as _integer_types
@@ -13,6 +14,12 @@
     path_string as _path_string)
 
 
+warnings.warn(
+    "OpenSSL.rand is deprecated - you should use os.urandom instead",
+    DeprecationWarning, stacklevel=3
+)
+
+
 class Error(Exception):
     """
     An error occurred in an :mod:`OpenSSL.rand` API.