micro-optimization: increase our float parsing speed by Nx

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2020-07-15.22:21:55.444>
labels = ['interpreter-core', 'type-feature', '3.10']
title = 'micro-optimization: increase our float parsing speed by Nx'
updated_at = <Date 2020-10-19.17:27:20.343>
user = 'https://github.com/gpshead'

bugs.python.org fields:

activity = <Date 2020-10-19.17:27:20.343>
actor = 'pitrou'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Interpreter Core']
creation = <Date 2020-07-15.22:21:55.444>
creator = 'gregory.p.smith'
dependencies = []
files = []
hgrepos = []
issue_num = 41310
keywords = []
message_count = 6.0
messages = ['373730', '373743', '373760', '373769', '373775', '378969']
nosy_count = 7.0
nosy_names = ['tim.peters', 'rhettinger', 'gregory.p.smith', 'mark.dickinson', 'pitrou', 'eric.smith', 'serhiy.storchaka']
pr_nums = []
priority = 'low'
resolution = None
stage = 'needs patch'
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue41310'
versions = ['Python 3.10']