python37Packages.cryptography: 2.6.1 -> 2.7

Changelog:
https://cryptography.io/en/latest/changelog/#v2-7

Important changes:
- BACKWARDS INCOMPATIBLE: We no longer distribute 32-bit manylinux1
  wheels. Continuing to produce them was a maintenance burden.
- BACKWARDS INCOMPATIBLE: Removed the
  cryptography.hazmat.primitives.mac.MACContext interface.
  The CMAC and HMAC APIs have not changed, but they are no longer
  registered as MACContext instances.