Add minimum supported python (#1044)

setup.py

@@ -43,6 +43,7 @@ setup(
     packages=find_packages(exclude=("tests",)),
     package_data={"datasette": ["templates/*.html"]},
     include_package_data=True,
+    python_requires=">=3.6",
     install_requires=[
         "asgiref>=3.2.10,<3.4.0",
         "click~=7.1.1",