File mode should be a constant

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2018-01-24.17:31:36.273>
labels = ['type-feature', '3.9', 'expert-IO']
title = 'File mode should be a constant'
updated_at = <Date 2019-05-30.23:07:00.463>
user = 'https://github.com/nagayev'

bugs.python.org fields:

activity = <Date 2019-05-30.23:07:00.463>
actor = 'berker.peksag'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['IO']
creation = <Date 2018-01-24.17:31:36.273>
creator = 'nagayev'
dependencies = []
files = []
hgrepos = []
issue_num = 32655
keywords = []
message_count = 4.0
messages = ['310620', '310622', '310667', '344010']
nosy_count = 5.0
nosy_names = ['pitrou', 'r.david.murray', 'berker.peksag', 'nitishch', 'nagayev']
pr_nums = []
priority = 'normal'
resolution = None
stage = 'test needed'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue32655'
versions = ['Python 3.9']