Fix typing in rfc822_formatted_time (aio-libs#3927)

- Add type annotations to global variables
- Fix handling of unused return values from time.gmtime
- Silence mypy error on return line (it's not yet capable of deducing
  that _cached_formatted_datetime is always defined at the end of the
  function)

aiohttp/helpers.py

@@ -455,8 +455,8 @@ def is_ip_address(
     return is_ipv4_address(host) or is_ipv6_address(host)
 
 
-_cached_current_datetime = None
-_cached_formatted_datetime = None
+_cached_current_datetime: Optional[int] = None
+_cached_formatted_datetime: Optional[str] = None
 
 
 def rfc822_formatted_time() -> str:
@@ -473,12 +473,12 @@ def rfc822_formatted_time() -> str:
                       "Jan", "Feb", "Mar", "Apr", "May", "Jun",
                       "Jul", "Aug", "Sep", "Oct", "Nov", "Dec")
 
-        year, month, day, hh, mm, ss, wd, y, z = time.gmtime(now)  # type: ignore  # noqa
+        year, month, day, hh, mm, ss, wd, *_ = time.gmtime(now)
         _cached_formatted_datetime = "%s, %02d %3s %4d %02d:%02d:%02d GMT" % (
             _weekdayname[wd], day, _monthname[month], year, hh, mm, ss
         )
         _cached_current_datetime = now
-    return _cached_formatted_datetime
+    return _cached_formatted_datetime  # type: ignore
 
 
 def _weakref_handle(info):  # type: ignore