python37Packages.cryptography: 2.5 -> 2.6.1

Changelog:
https://cryptography.io/en/latest/changelog/#v2-6-1

Important changes:
- BACKWARDS INCOMPATIBLE: Removed
  cryptography.hazmat.primitives.asymmetric.utils.encode_rfc6979_signature
  and
  cryptography.hazmat.primitives.asymmetric.utils.decode_rfc6979_signature,
  which had been deprecated for nearly 4 years. Use
  encode_dss_signature() and decode_dss_signature() instead.
- BACKWARDS INCOMPATIBLE: Removed cryptography.x509.Certificate.serial,
  which had been deprecated for nearly 3 years. Use serial_number
  instead.

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

@@ -7,7 +7,7 @@ buildPythonPackage rec {
 
   src = fetchPypi {
     inherit pname version;
-    sha256 = "15qfl3pnw2f11r0z0zhwl56f6pb60ysav8fxmpnz5p80cfwljdik";
+    sha256 = "1bsqcv3h49dzqnyn29ijq8r7k1ra8ikl1y9qcpcns9nbvhaq3wq3";
   };
 
   # No tests included