Bump cryptography from 2.7 to 2.8
Bump cryptography from 2.7 to 2.8

Bumps [cryptography](https://github.com/pyca/cryptography) from 2.7 to 2.8.
- [Release notes](https://github.com/pyca/cryptography/releases)
- [Changelog](https://github.com/pyca/cryptography/blob/master/CHANGELOG.rst)
- [Commits](https://github.com/pyca/cryptography/compare/2.7...2.8)

Signed-off-by: dependabot-preview[bot] <5bdcd3c0d4d24ae3e71b3b452a024c6324c7e4bb@dependabot.com>
aio-libs/aiohttp_security