Remove chardet, pytest-pep8 from dependencies, update music21 and fix haydn_op20 loader

[yujin-kimmm]

Closes #703, Closes #704

Description

Currently CI was failing because of chardet and music21.

Recent updates in chardet changed its encoding prediction, causing unicode extraction test to fail.
Encoding issue was found from irmas and goodsounds datasets, however, those two datasets can be encoded well with UTF-8. Therefore, fallback to chardet prediction in extractall_unicode is unnecessary.

Recent updates in music21 changed the API: score.flat() -> score.flatten(), which breaks the test.

++ 0629

pytest loads the pytest-pep8 automatically, which is no longer compatible with pytest 9, and it rejects its outdate pytest_collect_file(path, parent) hook signature.
This makes pytest fail before any tests run.

Mirdata already uses flake8 for style checks, and currently, test does not use pytest-pep8. So pytest-pep8 is no longer needed.

Changes

• Remove Chardet from dependencies, remove the encoding prediction fallback in extractall_unicode
• Update music21 to support newer version
• Fix haydn_op20 loader to support the new music21 API.

++0629

• Remove pytest-pep8 from pyproject.toml and CI environment.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.13%. Comparing base (5f51224) to head (8d4fc19).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #706      +/-   ##
==========================================
+ Coverage   97.12%   97.13%   +0.01%     
==========================================
  Files          71       71              
  Lines        7824     7828       +4     
==========================================
+ Hits         7599     7604       +5     
+ Misses        225      224       -1     

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