Tests: Replace gunicorn with waitress (works on Windows too)

setup.py

@@ -36,7 +36,6 @@ For further information, please visit the `Radicale Website
 
 """
 
-import os
 import sys
 
 from setuptools import find_packages, setup
@@ -53,9 +52,7 @@ WEB_FILES = ["web/internal_data/css/icon.png",
 needs_pytest = {"pytest", "test", "ptr"}.intersection(sys.argv)
 pytest_runner = ["pytest-runner"] if needs_pytest else []
 tests_require = ["pytest-runner", "pytest", "pytest-cov", "pytest-flake8",
-                 "pytest-isort"]
-if os.name == "posix":
-    tests_require.append("gunicorn")
+                 "pytest-isort", "waitress"]
 
 setup(
     name="Radicale",