build: Update tornado version in setup.py to match code check.

c06bf06 set the minimum version to 6, but setup.py was not updated to
match.

setup.py

@@ -129,7 +129,7 @@ def get_file_text(file_name):
         "sqlalchemy": ["sqlalchemy>=1.2"],
         "starlette": ["starlette>=0.19.1"],
         "starlite": ["starlite>=1.48"],
-        "tornado": ["tornado>=5"],
+        "tornado": ["tornado>=6"],
     },
     classifiers=[
         "Development Status :: 5 - Production/Stable",