Better `TypeError` message when a string is indexed using a non int

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-09-24.16:33:24.005>
labels = ['type-feature', 'library', '3.9']
title = 'Better `TypeError` message when a string is indexed using a non int'
updated_at = <Date 2021-09-24.17:28:19.493>
user = 'https://bugs.python.org/Rin'

bugs.python.org fields:

activity = <Date 2021-09-24.17:28:19.493>
actor = 'xtreak'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-09-24.16:33:24.005>
creator = 'Rin'
dependencies = []
files = []
hgrepos = []
issue_num = 45284
keywords = []
message_count = 2.0
messages = ['402577', '402578']
nosy_count = 2.0
nosy_names = ['xtreak', 'Rin']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue45284'
versions = ['Python 3.9']