build(deps-dev): update approvaltests requirement

Updates the requirements on [approvaltests](https://github.com/approvals/ApprovalTests.Python) to permit the latest version.
- [Release notes](https://github.com/approvals/ApprovalTests.Python/releases)
- [Commits](approvals/ApprovalTests.Python@v18.0.5...v18.1.0)

---
updated-dependencies:
- dependency-name: approvaltests
  dependency-version: 18.1.0
  dependency-type: direct:development
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

requirements-dev.txt

@@ -1,5 +1,5 @@
 pytest >= 9.0.3
-approvaltests >= 18.0.5
+approvaltests >= 18.1.0
 mypy >= 2.1.0
 pytest-approvaltests >= 0.2.4
 invoke >= 3.0.3