Warn when the user passes a TZ-naive datetime object

TZ-naive datetime objects are looking for trouble.  We would prefer to
do all comparisons and work using TZ-aware datetime objects.

pgpy/packet/packets.py

@@ -7,6 +7,7 @@ import copy
 import hashlib
 import os
 import re
+import warnings
 
 from datetime import datetime, timezone
 
@@ -763,6 +764,8 @@ class PubKeyV4(PubKey):
 
     @created.register(datetime)
     def created_datetime(self, val):
+        if val.tzinfo is None:
+            warnings.warn("Passing TZ-naive datetime object to PubKeyV4 packet")
         self._created = val
 
     @created.register(int)
@@ -1183,6 +1186,8 @@ class LiteralData(Packet):
 
     @mtime.register(datetime)
     def mtime_datetime(self, val):
+        if val.tzinfo is None:
+            warnings.warn("Passing TZ-naive datetime object to LiteralData packet")
         self._mtime = val
 
     @mtime.register(int)

pgpy/packet/subpackets/signature.py

@@ -4,6 +4,7 @@ Signature SubPackets
 """
 import binascii
 import calendar
+import warnings
 
 from datetime import datetime
 from datetime import timedelta
@@ -230,6 +231,8 @@ class CreationTime(Signature):
 
     @created.register(datetime)
     def created_datetime(self, val):
+        if val.tzinfo is None:
+            warnings.warn("Passing TZ-naive datetime object to CreationTime subpacket")
         self._created = val
 
     @created.register(int)