Add type checking with mypy

https://jira.usit.uio.no/browse/BOTTINT-190

• Remove CLI program
• Remove Client.return_objects option
• Remove Client.mock option
• Remove some unused functions
• Move linter invocations to tox
• Add flake8 linter
• Rewrite AnsattPatch class (not based on Ansatt)
• Remove pytest from setup.py (deprecated)
• Remove support for python 3.6