Convert manual creation/mod times to UTC as well

In line with the previous commit for consistency, convert the date
strings from --creationdate and --moddate to UTC as well if a timezone
is specified.

src/img2pdf.py

@@ -722,7 +722,8 @@ class pdfdoc(object):
             self.writer.docinfo = PdfDict(indirect=True)
 
         def datetime_to_pdfdate(dt):
-            return dt.strftime("%Y%m%d%H%M%S%z")
+            dt_utc = dt.astimezone(tz=timezone.utc)
+            return dt_utc.strftime("%Y%m%d%H%M%S%z")
 
         for k in ["Title", "Author", "Creator", "Producer", "Subject"]:
             v = locals()[k.lower()]
@@ -732,7 +733,7 @@ class pdfdoc(object):
                 v = PdfString.encode(v)
             self.writer.docinfo[getattr(PdfName, k)] = v
 
-        now = datetime.now(tz=timezone.utc)
+        now = datetime.now().astimezone()
         for k in ["CreationDate", "ModDate"]:
             v = locals()[k.lower()]
             if v is None and nodate:
@@ -752,7 +753,8 @@ class pdfdoc(object):
                 )
 
         def datetime_to_xmpdate(dt):
-            return dt.strftime("%Y-%m-%dT%H:%M:%S%z")
+            dt_utc = dt.astimezone(tz=timezone.utc)
+            return dt_utc.strftime("%Y-%m-%dT%H:%M:%S%z")
 
         self.xmp = b"""<?xpacket begin='\xef\xbb\xbf' id='W5M0MpCehiHzreSzNTczkc9d'?>
 <x:xmpmeta xmlns:x='adobe:ns:meta/' x:xmptk='XMP toolkit 2.9.1-13, framework 1.6'>