Fix when parsing tz offsets microseconds shorter than 6 (#4781)

As the remainder was directly parsed as an int, strings like
.600 were parsed as 600 microseconds rather than milliseconds.

Author: mariocj89

Lib/_strptime.py

@@ -470,7 +470,10 @@ def _strptime(data_string, format="%a %b %d %H:%M:%S %Y"):
                 minutes = int(z[3:5])
                 seconds = int(z[5:7] or 0)
                 gmtoff = (hours * 60 * 60) + (minutes * 60) + seconds
-                gmtoff_fraction = int(z[8:] or 0)
+                gmtoff_remainder = z[8:]
+                # Pad to always return microseconds.
+                gmtoff_remainder_padding = "0" * (6 - len(gmtoff_remainder))
+                gmtoff_fraction = int(gmtoff_remainder + gmtoff_remainder_padding)
                 if z.startswith("-"):
                     gmtoff = -gmtoff
                     gmtoff_fraction = -gmtoff_fraction

Lib/test/test_strptime.py

@@ -345,6 +345,9 @@ def test_offset(self):
         (*_, offset), _, offset_fraction = _strptime._strptime("-01:30:30.000001", "%z")
         self.assertEqual(offset, -(one_hour + half_hour + half_minute))
         self.assertEqual(offset_fraction, -1)
+        (*_, offset), _, offset_fraction = _strptime._strptime("+01:30:30.001", "%z")
+        self.assertEqual(offset, one_hour + half_hour + half_minute)
+        self.assertEqual(offset_fraction, 1000)
         (*_, offset), _, offset_fraction = _strptime._strptime("Z", "%z")
         self.assertEqual(offset, 0)
         self.assertEqual(offset_fraction, 0)