email: ValueError in get_section when parsing header with non-ASCII digit

[The-Compiler]

BPO	42946
Nosy	@warsaw, @bitdancer, @maxking, @The-Compiler

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2021-01-17.17:14:51.665>
labels = ['type-bug', '3.8', '3.9', '3.10', '3.7', 'library']
title = 'email: ValueError in get_section when parsing header with non-ASCII digit'
updated_at = <Date 2021-01-17.17:14:51.665>
user = 'https://github.com/The-Compiler'

bugs.python.org fields:

activity = <Date 2021-01-17.17:14:51.665>
actor = 'The Compiler'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2021-01-17.17:14:51.665>
creator = 'The Compiler'
dependencies = []
files = []
hgrepos = []
issue_num = 42946
keywords = []
message_count = 1.0
messages = ['385162']
nosy_count = 4.0
nosy_names = ['barry', 'r.david.murray', 'maxking', 'The Compiler']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42946'
versions = ['Python 3.6', 'Python 3.7', 'Python 3.8', 'Python 3.9', 'Python 3.10']

[The-Compiler]

Found mostly by accident:

>>> import email.headerregistry
>>> reg = email.headerregistry.HeaderRegistry()
>>> h = reg('Content-Disposition', 'inline; 0*²')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.10/email/headerregistry.py", line 608, in __call__
    return self[name](name, value)
  File "/usr/lib/python3.10/email/headerregistry.py", line 196, in __new__
    cls.parse(value, kwds)
  File "/usr/lib/python3.10/email/headerregistry.py", line 452, in parse
    kwds['parse_tree'] = parse_tree = cls.value_parser(value)
  File "/usr/lib/python3.10/email/_header_value_parser.py", line 2705, in parse_content_disposition_header
    disp_header.append(parse_mime_parameters(value[1:]))
  File "/usr/lib/python3.10/email/_header_value_parser.py", line 2569, in parse_mime_parameters
    token, value = get_parameter(value)
  File "/usr/lib/python3.10/email/_header_value_parser.py", line 2431, in get_parameter
    token, value = get_section(value)
  File "/usr/lib/python3.10/email/_header_value_parser.py", line 2384, in get_section
    section.number = int(digits)
ValueError: invalid literal for int() with base 10: '²'

This probably happens because:

>>> '²'.isdigit()
True
>>> int('²')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: '²'