bpo-33812: Corrected astimezone for naive datetimes. (GH-7578) (GH-7600)

A datetime object d is aware if d.tzinfo is not None and
d.tzinfo.utcoffset(d) does  not return None. If d.tzinfo is None,
or if d.tzinfo is not None but d.tzinfo.utcoffset(d) returns None,
 d is naive.

This commit ensures that instances with non-None d.tzinfo, but
d.tzinfo.utcoffset(d) returning None are treated as naive.

In addition, C acceleration code will raise TypeError if
d.tzinfo.utcoffset(d) returns an object with the type other than
timedelta.

* Updated the documentation.

Assume that the term "naive" is defined elsewhere and remove the
not entirely correct clarification.  Thanks, Tim.
(cherry picked from commit 877b23202b7e7d4f57b58504fd0eb886e8c0b377)

Co-authored-by: Alexander Belopolsky <abalkin@users.noreply.github.com>
diff --git a/Lib/datetime.py b/Lib/datetime.py
index 8fa18a7..dd6eca9 100644
--- a/Lib/datetime.py
+++ b/Lib/datetime.py
@@ -1780,14 +1780,17 @@
         mytz = self.tzinfo
         if mytz is None:
             mytz = self._local_timezone()
+            myoffset = mytz.utcoffset(self)
+        else:
+            myoffset = mytz.utcoffset(self)
+            if myoffset is None:
+                mytz = self.replace(tzinfo=None)._local_timezone()
+                myoffset = mytz.utcoffset(self)
 
         if tz is mytz:
             return self
 
         # Convert self to UTC, and attach the new time zone object.
-        myoffset = mytz.utcoffset(self)
-        if myoffset is None:
-            raise ValueError("astimezone() requires an aware datetime")
         utc = (self - myoffset).replace(tzinfo=tz)
 
         # Convert from UTC to tz's local time.