ElementTree: wrong XML prolog for the utf-8-sig encoding

[pepr]

BPO	46598
Nosy	@pepr, @scoder
PRs	gh-90756: Fix ElementTree.write XML prolog for encoding 'utf-8-sig' #31043

^{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 2022-02-01.11:00:34.033>
labels = ['library', '3.10', '3.11']
title = 'ElementTree: wrong XML prolog for the utf-8-sig encoding'
updated_at = <Date 2022-02-01.16:40:13.781>
user = 'https://github.com/pepr'

bugs.python.org fields:

activity = <Date 2022-02-01.16:40:13.781>
actor = 'ned.deily'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2022-02-01.11:00:34.033>
creator = 'prikryl'
dependencies = []
files = []
hgrepos = []
issue_num = 46598
keywords = []
message_count = 1.0
messages = ['412247']
nosy_count = 3.0
nosy_names = ['prikryl', 'scoder', 'eli.bendersky']
pr_nums = ['31043']
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = None
url = 'https://bugs.python.org/issue46598'
versions = ['Python 3.10', 'Python 3.11']

[pepr]

When ElementTree object is to be written to the file, and when BOM is needed, the 'utf-8-sig' can be used for the purpose. However, the XML prolog then looks like...

\<?xml version='1.0' encoding='utf-8-sig'?\>

... and that encoding in the prolog makes no sense. Therefore,
the utf-8-sig is changed to utf-8 for the purpose.

To fix the situation, the following two lines should be added to
cpython/Lib/xml/etree/ElementTree.py

elif enc_lower == "utf-8-sig": declared_encoding = "utf-8"

just above the line 741 that says
write("<?xml version='1.0' encoding='%s'?>\n" % ( declared_encoding,))

I have already cloned the main branch, added the lines to https://github.com/pepr/cpython.git, and sent pull request.

I have tested the functionality locally with Python 3.10.2 (tags/v3.10.2:a58ebcc, Jan 17 2022, 14:12:15) [MSC v.1929 64 bit (AMD64)] on win32