Hardcode epoch constant

Rather than calculate it, hard code the Unix epoch as a timedelta
for Python 2.7 compatibility.

src/pytz/tzinfo.py

@@ -1,6 +1,6 @@
 '''Base classes and helpers for building zone specific tzinfo classes'''
 
-from datetime import datetime, timedelta, timezone, tzinfo
+from datetime import datetime, timedelta, tzinfo
 from bisect import bisect_right
 try:
     set
@@ -24,7 +24,8 @@ def memorized_timedelta(seconds):
         _timedelta_cache[seconds] = delta
         return delta
 
-_epoch = datetime.fromtimestamp(0, tz=timezone.utc).replace(tzinfo=None)
+
+_epoch = datetime(1970, 1, 1, 0, 0) # datetime.utcfromtimestamp(0)
 _datetime_cache = {0: _epoch}
 
 
@@ -39,6 +40,7 @@ def memorized_datetime(seconds):
         _datetime_cache[seconds] = dt
         return dt
 
+
 _ttinfo_cache = {}
 
 
@@ -55,6 +57,7 @@ def memorized_ttinfo(*args):
         _ttinfo_cache[args] = ttinfo
         return ttinfo
 
+
 _notime = memorized_timedelta(0)