Log Message:
Changes 0.25.0:
- More cleanups, removal of obsolete stuff, and moves towards py3k
  compatibility.
- Add support for EC.get_builtin_curves() and use it for testing.
- Enable AES CTR mode
- Bundle-in six module v. 1.10.0
- add rand_file_name and rand_status
- remove all LHASH fiddling
- Extend Travis and GitLab CI configuration to test also py3k (with
  allowed_failures) and CentOS6 (on GitLab CI).
- Add CONTRIBUTORS.rst. Thank you!
- Add PEP-484 type hints in comments to all Python files (except for
  tests)
- Use context managers for file handling wherever possible instead of
  leaking open file descriptors.
- Improve defaults handling for SSL_CTX_new().
- Fix PGP tests to actually run