Miscellaneous minor fixes

[joshuagl]

This PR splits out some of the unrelated fixes from #200

Description of the changes being introduced by the pull request:

• Update tested Python versions to match tuf
• Removed unused DecompressionError
• Fix misspelling in PULL_REQUEST_TEMPLATE
• Remove a duplicate import from test_interface.py

Please verify and check that the pull request fulfils the following
requirements:

• The code follows the Code Style Guidelines
• Tests have been added for the bug fix or new feature
• Docs have been added for the bug fix or new feature

[lukpueh]

Thanks for the fixes, @joshuagl! I permitted myself to update your PR so that your newly added tox test environments are also executed on travis. I hope you don't mind.