Refactor checking for substring in output

Update linters

Allow newer versions of FFPuppet (#97)

Other projects depend on Lithium and FFPuppet and this
will help avoid dependency issues.

Use digest instead of hexdigest when caching reduction attempts.

Search string must be encoded to bytes (#93)

Modernize the interestingness tests (#92)

* Modernize the interestingness tests

* Re-add module docstrings

* Remove license from __init__.py

* Raise parser error if command wasn't specified

* Change to relative import

* Re-add interesting text to log messages

* Explicitly check list length

* Add each test argument as a new list element

* Drop Python 3.7

Merge pull request #85 from MozillaSecurity/fix-type

Fix log_prefix type to be optional

Fix plural atom

Release 0.6.0:

* Add type annotations (thanks gkw!)
* Fix bugs found while adding type annotations