Python: Eastern Standard Time vs. Eastern Daylight Time

eastern_time.py

"""
https://en.wikipedia.org/wiki/Eastern_Time_Zone
"""

>>> import datetime, pytz

# 5 hour difference during Eastern Standard Time
>>> datetime.datetime(2018, 1, 25, 16, 26, tzinfo=pytz.timezone('US/Eastern')).hour
>>> 16

>>> datetime.datetime(2018, 1, 25, 16, 26, tzinfo=pytz.timezone('US/Eastern')).astimezone(pytz.utc).hour
>>> 21

# 4 hour difference during Eastern Daylight Time
>>> datetime.datetime(2018, 4, 25, 16, 26, tzinfo=pytz.timezone('US/Eastern')).hour
>>> 16

# So why does this not say 20?
>>> datetime.datetime(2018, 4, 25, 16, 26, tzinfo=pytz.timezone('US/Eastern')).astimezone(pytz.utc).hour
>>> 21