fix: add setuptools python_requires check

Author: Nicusor97

setup.py

@@ -17,6 +17,7 @@
     py_modules=["pytest_timeout"],
     entry_points={"pytest11": ["timeout = pytest_timeout"]},
     install_requires=["pytest>=5.0.0"],
+    python_requires='>=3.6',
     classifiers=[
         "Development Status :: 5 - Production/Stable",
         "Environment :: Console",