MAINT: Drop support for Python 3.6 and 3.7

Summary

For #125 (closed). Remove support for Python versions not longer present in the two most recent Ubuntu-LTS versions

Focus

Nothing added, just increased minimal Python version. Have a look at the TODO in connection.py.

Test Environment

Usual pipeline is sufficent; verify that tox doesn't even attempt unsupported Python versions.

Check List for the Author

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