Improve type signature for _RuleList

[mhils]

This PR improves the type signature for _RuleList to be more forgiving. Minimal repro:

from tornado.web import Application, RequestHandler

class SampleHandler(RequestHandler):
    def get(self):
        self.write("Hello world")

handlers = [
    (r"/", SampleHandler),
]

application = Application(handlers)

Before Patch:

$ mypy repro.py
repro.py:11: error: Argument 1 to "Application" has incompatible type "list[tuple[str, type[SampleHandler]]]"; expected "list[Rule | list[Any] | tuple[str | Matcher, Any] | tuple[str | Matcher, Any, dict[str, Any]] | tuple[str | Matcher, Any, dict[str, Any], str]] | None"  [arg-type]
repro.py:11: note: "List" is invariant -- see https://mypy.readthedocs.io/en/stable/common_issues.html#variance
repro.py:11: note: Consider using "Sequence" instead, which is covariant
Found 1 error in 1 file (checked 1 source file)

After Patch:

$ mypy repro.py
Success: no issues found in 1 source file

Thank you for maintaining tornado, we (@mitmproxy) are super happy users for over a decade! 😃✨🍰