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.