Upgrade cryptography to 2.8

[remeh]

In order to build the Agent with Python 3.8, we need a compatible cryptography and the compatibility starts with cryptography 2.8.

What does this PR do?

Upgrade cryptography to 2.8.

Motivation

In order to build the Agent with Python 3.8, we need a compatible cryptography and the compatibility starts with cryptography 2.8.

Additional Notes

Issue on the cryptography repo about the Python 3.8 support pyca/cryptography#5010

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached

[codecov]

Codecov Report

Merging #5047 into master will increase coverage by 0.59%.
The diff coverage is n/a.

Impacted Files	Coverage Δ
http_check/datadog_checks/http_check/__init__.py	100% <0%> (ø)	⬆️
...penmetrics/datadog_checks/openmetrics/__about__.py	100% <0%> (ø)	⬆️
php_fpm/tests/test_e2e.py	23.8% <0%> (ø)	⬆️
couchbase/tests/test_unit.py	100% <0%> (ø)	⬆️
aerospike/tests/common.py	100% <0%> (ø)	⬆️
coredns/tests/common.py	100% <0%> (ø)	⬆️
http_check/datadog_checks/http_check/adapters.py	38.63% <0%> (ø)	⬆️
openmetrics/datadog_checks/openmetrics/__init__.py	100% <0%> (ø)	⬆️
fluentd/datadog_checks/fluentd/fluentd.py	89.36% <0%> (ø)	⬆️
http_check/tests/conftest.py	69.56% <0%> (ø)	⬆️
... and 734 more