Ensure async https() requests are bounded in total time

according to the timeout [#978].

Unfortunately we do not currently have a good way to
make this guarantee for sync https() calls.

dns/_asyncbackend.py

@@ -94,3 +94,6 @@ async def sleep(self, interval):
 
     def get_transport_class(self):
         raise NotImplementedError
+
+    async def wait_for(self, awaitable, timeout):
+        raise NotImplementedError

dns/_asyncio_backend.py

@@ -270,3 +270,6 @@ def datagram_connection_required(self):
 
     def get_transport_class(self):
         return _HTTPTransport
+
+    async def wait_for(self, awaitable, timeout):
+        return await _maybe_wait_for(awaitable, timeout)

dns/_trio_backend.py

@@ -237,3 +237,10 @@ async def sleep(self, interval):
 
     def get_transport_class(self):
         return _HTTPTransport
+
+    async def wait_for(self, awaitable, timeout):
+        with _maybe_timeout(timeout):
+            return await awaitable
+        raise dns.exception.Timeout(
+            timeout=timeout
+        )  # pragma: no cover  lgtm[py/unreachable-statement]

dns/asyncquery.py

@@ -563,14 +563,14 @@ async def https(
                     "content-length": str(len(wire)),
                 }
             )
-            response = await the_client.post(
-                url, headers=headers, content=wire, timeout=timeout
+            response = await backend.wait_for(
+                the_client.post(url, headers=headers, content=wire), timeout
             )
         else:
             wire = base64.urlsafe_b64encode(wire).rstrip(b"=")
             twire = wire.decode()  # httpx does a repr() if we give it bytes
-            response = await the_client.get(
-                url, headers=headers, timeout=timeout, params={"dns": twire}
+            response = await backend.wait_for(
+                the_client.get(url, headers=headers, params={"dns": twire}), timeout
             )
 
     # see https://tools.ietf.org/html/rfc8484#section-4.2.1 for info about DoH