python37Packages.cryptography: 2.3.1 -> 2.4.2

Changelog:
https://cryptography.io/en/latest/changelog/#v2-4-2

Important changes:
- BACKWARDS INCOMPATIBLE: Dropped support for LibreSSL 2.4.x.
- Deprecated OpenSSL 1.0.1 support. OpenSSL 1.0.1 is no longer supported
  by the OpenSSL project. At this time there is no time table for
  dropping support, however we strongly encourage all users to upgrade
  or install cryptography from a wheel.