Update diff-cover requirement from <7.7 to <7.8 (#719)

Updates the requirements on [diff-cover](https://github.com/Bachmann1234/diff-cover) to permit the latest version.
- [Release notes](https://github.com/Bachmann1234/diff-cover/releases)
- [Changelog](https://github.com/Bachmann1234/diff_cover/blob/main/CHANGELOG)
- [Commits](Bachmann1234/diff_cover@v0.1.0...v7.7.0)

---
updated-dependencies:
- dependency-name: diff-cover
  dependency-type: direct:development
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

dev-requirements.txt

@@ -1,4 +1,4 @@
-diff_cover < 7.7
+diff_cover < 7.8
 pylint < 2.18
 pydocstyle < 6.4
 flake8 > 4