Treat default creation/mod dates as UTC (fixes #155)

Without passing a tzinfo object to `datetime.now()`, a "naive" datetime
object is created, which is not timezone-aware. To fix the default
date/time detection for non-UTC local timezones, pass
`datetime.timezone.utc` to convert the value to UTC and make the
datetime object "aware".

Also, adjust the strftime() wrappers to use the UTC offsets instead of a
literal `Z`; using the literal `Z` at the end appears to be valid
ISO-8601, but for some reason it does not successfully convert, whereas
the `%z` placeholder substitutes the UTC offset and successfully
converts.

src/img2pdf.py

@@ -37,6 +37,7 @@ if hasattr(GifImagePlugin, "LoadingStrategy"):
 # TiffImagePlugin.DEBUG = True
 from PIL.ExifTags import TAGS
 from datetime import datetime
+from datetime import timezone
 from jp2 import parsejp2
 from enum import Enum
 from io import BytesIO
@@ -721,7 +722,7 @@ class pdfdoc(object):
             self.writer.docinfo = PdfDict(indirect=True)
 
         def datetime_to_pdfdate(dt):
-            return dt.strftime("%Y%m%d%H%M%SZ")
+            return dt.strftime("%Y%m%d%H%M%S%z")
 
         for k in ["Title", "Author", "Creator", "Producer", "Subject"]:
             v = locals()[k.lower()]
@@ -731,7 +732,7 @@ class pdfdoc(object):
                 v = PdfString.encode(v)
             self.writer.docinfo[getattr(PdfName, k)] = v
 
-        now = datetime.now()
+        now = datetime.now(tz=timezone.utc)
         for k in ["CreationDate", "ModDate"]:
             v = locals()[k.lower()]
             if v is None and nodate:
@@ -751,7 +752,7 @@ class pdfdoc(object):
                 )
 
         def datetime_to_xmpdate(dt):
-            return dt.strftime("%Y-%m-%dT%H:%M:%SZ")
+            return dt.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'>