Add support for PHPUnit 8

[owenvoke]

Description

This adds support for PHPUnit 8 and drops PHPUnit 6 support.

Types of changes

Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

Put an x in all the boxes that apply:

• I have read the CONTRIBUTING document.
• My pull request addresses exactly one patch/feature.
• I have created a branch for this patch/feature.

If you're unsure about any of these, don't hesitate to ask. We're here to help!

[codecov]

Codecov Report

Merging #1 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##             develop     #1   +/-   ##
========================================
  Coverage        100%   100%           
  Complexity        93     93           
========================================
  Files              5      5           
  Lines            207    207           
========================================
  Hits             207    207

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