Update sphinx requirement from >=1.4 to >=8.1.3

Updates the requirements on [sphinx](https://github.com/sphinx-doc/sphinx) to permit the latest version.
- [Release notes](https://github.com/sphinx-doc/sphinx/releases)
- [Changelog](https://github.com/sphinx-doc/sphinx/blob/v8.1.3/CHANGES.rst)
- [Commits](sphinx-doc/sphinx@v1.4...v8.1.3)

---
updated-dependencies:
- dependency-name: sphinx
  dependency-version: 8.1.3
  dependency-type: direct:production
...

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

Author: dependabot[bot]

test-requirements-asyncio.txt

@@ -4,4 +4,4 @@ py>=1.4.31
 pytest
 pytest-cov
 randomize>=0.13
-sphinx>=1.4 # BSD
+sphinx>=8.1.3 # BSD

test-requirements.txt

@@ -4,7 +4,7 @@ pytest
 pytest-cov
 pluggy>=0.3.1
 randomize>=0.13
-sphinx>=1.4 # BSD
+sphinx>=8.1.3 # BSD
 sphinx_markdown_tables
 pycodestyle
 autopep8