More precise annotations of utils.timezone functions that return inst…

…ances of tzinfo subclasses. (#209)

* fix annotations of utils.timezone

* use intermediary tzinfo subclass exposed in pytz typeshed

* fix annotations of get_fixed_timezone as it returns datetime.timezone in Django 2.2

* add explanatory comment to get_current_timezone annotations

* black utils.timezone.pyi

django-stubs/utils/timezone.pyi

@@ -1,8 +1,10 @@
 import types
 from contextlib import ContextDecorator
-from datetime import date, datetime as datetime, time, timedelta as timedelta, tzinfo as tzinfo
+from datetime import date, datetime as datetime, time, timedelta as timedelta, tzinfo as tzinfo, timezone
 from typing import Optional, Union, Type
 
+from pytz import BaseTzInfo
+
 _AnyTime = Union[time, datetime]
 
 class UTC(tzinfo):
@@ -30,10 +32,14 @@ class LocalTimezone(ReferenceLocalTimezone):
 
 utc: UTC = ...
 
-def get_fixed_timezone(offset: Union[timedelta, int]) -> tzinfo: ...
-def get_default_timezone() -> tzinfo: ...
+def get_fixed_timezone(offset: Union[timedelta, int]) -> timezone: ...
+def get_default_timezone() -> BaseTzInfo: ...
 def get_default_timezone_name() -> str: ...
-def get_current_timezone() -> tzinfo: ...
+
+# Strictly speaking, it is possible to activate() a non-pytz timezone,
+# in which case BaseTzInfo is incorrect. However, this is unlikely,
+# so we use it anyway, to keep things ergonomic for most users.
+def get_current_timezone() -> BaseTzInfo: ...
 def get_current_timezone_name() -> str: ...
 def activate(timezone: Union[tzinfo, str]) -> None: ...
 def deactivate() -> None: ...