Improve test client type stubs (#322)

* Added missing `follow: bool` argument to Client request methods.
* Annotated return types as `HttpResponse` instead of `Any`.
* Made `Client` class inherit from `RequestFactory`, as it does in Django.
* Changed `json()` return type to Any, as it can also be a list.

Wrt (2), these return types were reverted from `HttpResponse` to `Any`
in commit 287c64d6fb. But I suspect that
was simply to silence mypy warnings about "incompatible with supertype
RequestFactory", not because there were any issues with the annotation.

Note that `Client.request()` monkey-patches the `HttpResponse` object
with some additional attributes. Those attributes were already annotated
before, I reordered and added additional comments to make it clear where
they come from.

test-data/typecheck/test/test_testcase.yml

@@ -0,0 +1,12 @@
+-   case: testcase_client_attr
+    main: |
+      from django.test.testcases import TestCase
+
+      class ExampleTestCase(TestCase):
+          def test_method(self) -> None:
+              reveal_type(self.client)  # N: Revealed type is 'django.test.client.Client'
+              resp = self.client.post('/url', {'doit': 'srs'}, 'application/json', False, True, extra='value')
+              reveal_type(resp.status_code)   # N: Revealed type is 'builtins.int'
+              # Attributes monkey-patched by test Client class:
+              resp.json()
+              reveal_type(resp.wsgi_request)  # N: Revealed type is 'django.core.handlers.wsgi.WSGIRequest'