Improve stubs for runserver management command. (#1037)

This adds type annotation for `run`, `get_handler`, `inner_run` and
`server_cls`.

Source code:
https://github.com/django/django/blob/03eec9ff6cc78e7c1bcf88bb76ecd11f0d433c72/django/core/management/commands/runserver.py

django-stubs/core/management/commands/runserver.pyi

@@ -1,7 +1,15 @@
+from typing import Any, Type
+
+from django.core.handlers.wsgi import WSGIHandler
 from django.core.management.base import BaseCommand
+from django.core.servers.basehttp import WSGIServer
 
 class Command(BaseCommand):
     default_addr: str = ...
     default_addr_ipv6: str = ...
     default_port: str = ...
     protocol: str = ...
+    server_cls: Type[WSGIServer] = ...
+    def run(self, **options: Any) -> None: ...
+    def get_handler(self, *args: Any, **options: Any) -> WSGIHandler: ...
+    def inner_run(self, *args: Any, **options: Any) -> None: ...