pip(deps): update attrs requirement from ~=21.2 to ~=21.4 (#199)

Updates the requirements on [attrs](https://github.com/python-attrs/attrs) to permit the latest version.
- [Release notes](https://github.com/python-attrs/attrs/releases)
- [Changelog](https://github.com/python-attrs/attrs/blob/main/CHANGELOG.rst)
- [Commits](python-attrs/attrs@21.2.0...21.4.0)

---
updated-dependencies:
- dependency-name: attrs
  dependency-type: direct:production
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
Co-authored-by: Nicholas Cilfone <nicholas.cilfone@fmr.com>

REQUIREMENTS.txt

@@ -1,4 +1,4 @@
-attrs~=21.2
+attrs~=21.4
 GitPython~=3.1
 pytomlpp~=1.0
 pyYAML~=5.4