bpo-29919: Remove unused imports found by pyflakes (#137)

Make also minor PEP8 coding style fixes on modified imports.
diff --git a/Tools/tz/zdump.py b/Tools/tz/zdump.py
index f94b483..39de0a4 100644
--- a/Tools/tz/zdump.py
+++ b/Tools/tz/zdump.py
@@ -3,7 +3,7 @@
 import struct
 from array import array
 from collections import namedtuple
-from datetime import datetime, timedelta
+from datetime import datetime
 
 ttinfo = namedtuple('ttinfo', ['tt_gmtoff', 'tt_isdst', 'tt_abbrind'])