chore: update sphinx requirement from ==3.* to >=3,<5

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/4.x/CHANGES)
- [Commits](sphinx-doc/sphinx@v3.0.0b1...v4.0.0)

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

Author: dependabot[bot]

setup.py

@@ -45,7 +45,7 @@ def find_version(*file_paths):
         "guzzle-sphinx-theme==0.*",
         "m2r2==0.*",
         "pytest-cov>=2,<4",
-        "sphinx==3.*",
+        "sphinx>=3,<5",
         "sphinxcontrib-log-cabinet==1.*",
         "sphinxcontrib-websupport==1.*",
         "twine==3.*",