python37Packages.cryptography: Improve the test vectors integration

This should make the management easier. The package cryptography_vectors
contains the test vectors for cryptography and should therefore always
have the same version. By linking the version of cryptography_vectors to
cryptography, this simply cannot be forgotten.

pkgs/development/python-modules/cryptography/default.nix

@@ -20,9 +20,8 @@
 }:
 
 buildPythonPackage rec {
-  # also bump cryptography_vectors
   pname = "cryptography";
-  version = "2.5";
+  version = "2.5"; # Also update the hash in vectors.nix
 
   src = fetchPypi {
     inherit pname version;

pkgs/development/python-modules/cryptography/vectors.nix

@@ -1,9 +1,9 @@
-{ buildPythonPackage, fetchPypi, lib }:
+{ buildPythonPackage, fetchPypi, lib, cryptography }:
 
 buildPythonPackage rec {
-  # also bump cryptography
   pname = "cryptography_vectors";
-  version = "2.5";
+  # The test vectors must have the same version as the cryptography package:
+  version = cryptography.version;
 
   src = fetchPypi {
     inherit pname version;