[pixiv] work around strptime limitations in Python < 3.7

"%z" doesn't allow a colon separator in older Python versions:
    - "+0900" is OK
    - "+09:00" raises an exception

gallery_dl/extractor/pixiv.py

@@ -42,7 +42,7 @@ class PixivExtractor(Extractor):
             del work["meta_pages"]
             work["num"] = ""
             work["tags"] = [tag["name"] for tag in work["tags"]]
-            work["date"] = text.parse_datetime(work["create_date"])
+            work["date"] = text.parse_datetime(work["create_date"][:-3] + "00")
             work.update(metadata)
 
             yield Message.Directory, work

test/test_text.py

@@ -353,11 +353,11 @@ class TestText(unittest.TestCase):
     def test_parse_datetime(self, f=text.parse_datetime):
         null = datetime.datetime.utcfromtimestamp(0)
 
-        self.assertEqual(f("1970-01-01T00:00:00+00:00"), null)
-        self.assertEqual(f("1970.01.01", "%Y.%m.%d")   , null)
+        self.assertEqual(f("1970-01-01T00:00:00+0000"), null)
+        self.assertEqual(f("1970.01.01", "%Y.%m.%d")  , null)
 
         self.assertEqual(
-            f("2019-05-07T21:25:02+09:00"),
+            f("2019-05-07T21:25:02+0900"),
             datetime.datetime(2019, 5, 7, 12, 25, 2),
         )