Accelerate string.Template by using formatted string literals

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-09-29.09:01:25.978>
labels = ['3.7', 'library', 'performance']
title = 'Accelerate string.Template by using formatted string literals'
updated_at = <Date 2016-09-29.09:02:14.892>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2016-09-29.09:02:14.892>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2016-09-29.09:01:25.978>
creator = 'serhiy.storchaka'
dependencies = []
files = ['44878']
hgrepos = []
issue_num = 28309
keywords = ['patch']
message_count = 1.0
messages = ['277690']
nosy_count = 3.0
nosy_names = ['georg.brandl', 'eric.smith', 'serhiy.storchaka']
pr_nums = []
priority = 'low'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue28309'
versions = ['Python 3.7']