Update autotests - Githubissues

Purpose of proposed changes

Keep CI up to date

Essential steps taken

Add Python 3.9 to test env matrix
Add build dependencies for cryptography package (required only for autotests)
Switch coverage and linting tests to latest python version
Fix linting to adhere latest pylint requirements
Switch test env distro to Ubuntu 18.04 and make according changes in test env configs
Drop tests with uvloop on Python<3.7 since uvloop upstream doesn't seem to support older Python versions anymore

Snawoot / postfix-mta-sts-resolver