parser: Ignore UTF-8 BOM characters

[vimpostor]

Some particularly badly written ics files can start with the UTF-8 BOM character.

While they are not strictly according to the spec, it is easy to add support for them.
Before this patch, icalendar would choke on these files with:

Traceback (most recent call last):
  File "/usr/lib/python3.12/site-packages/icalendar/parser.py", line 339, in parts
    validate_token(name)
  File "/usr/lib/python3.12/site-packages/icalendar/parser.py", line 128, in validate_token
    raise ValueError(name)
ValueError: BEGIN

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/bin/icalendar", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/usr/lib/python3.12/site-packages/icalendar/cli.py", line 82, in main
    calendar = Calendar.from_ical(f.read())
               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/lib/python3.12/site-packages/icalendar/cal.py", line 331, in from_ical
    name, params, vals = line.parts()
                         ^^^^^^^^^^^^
  File "/usr/lib/python3.12/site-packages/icalendar/parser.py", line 353, in parts
    raise ValueError(
ValueError: Content line could not be parsed into parts: 'BEGIN:VCALENDAR': BEGIN

After this patch, icalendar is able to parse these ics files.

Let me know, if you prefer another solution to this.

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📚 Documentation preview 📚: https://icalendar--700.org.readthedocs.build/

[coveralls]

Pull Request Test Coverage Report for Build 10394696884

Details

• 4 of 5 (80.0%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 97.493%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/icalendar/cli.py	0	1	0.0%

Totals
Change from base Build 10134522684:	0.001%
Covered Lines:	3178
Relevant Lines:	3256

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💛 - Coveralls

[niccokunzmann]

Nice! I merge this right away! Thanks!

[vimpostor]

Nice! I merge this right away! Thanks!

Thanks, that was fast! :)