Preserve form feed blank lines

[lin-hongkuan]

str.splitlines() treats form feed (\f, U+000C) as a line boundary, so parsing a file with a form-feed-only blank line drops the character before output is generated. Later, the import-tail blank-line cleanup also treats any strip()-empty line as removable.

This changes parsing to split only on actual newline sequences (\r\n, \r, \n) and keeps import-tail cleanup from removing blank lines that contain form feed. That preserves valid Python blank lines while keeping the existing cleanup for ordinary whitespace-only lines.

fixes #2562

Tests:

• py -3 -m pytest tests/unit/test_regressions.py::test_form_feed_blank_line_not_removed_issue_2562 tests/unit/test_api.py::test_sort_code_string_mixed_newlines tests/unit/test_isort.py::test_ensure_line_endings_are_preserved_issue_493 tests/unit/test_isort.py::test_line_endings_are_detected_ignoring_whitespace -q
• py -3 -m pytest tests/unit/test_regressions.py tests/unit/test_api.py tests/unit/test_isort.py -q
• py -3 -m ruff check isort/parse.py isort/output.py tests/unit/test_regressions.py --ignore PLC0207
• py -3 -m mypy isort/parse.py isort/output.py

Note: a full tests/unit run produced 569 passed / 5 skipped before failing 4 environment-dependent tests unrelated to this change (example profile/sort plugin entry points and a Windows diff fixture side effect).

[bignose-debian]

@lin-hongkuan:

This changes parsing to split only on actual newline sequences (\r\n, \r, \n) and keeps import-tail cleanup from removing blank lines that contain form feed. That preserves valid Python blank lines while keeping the existing cleanup for ordinary whitespace-only lines.

Thank you for creating this well-targeted change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.16%. Comparing base (0740d0c) to head (47cf7ae).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2565   +/-   ##
=======================================
  Coverage   99.16%   99.16%           
=======================================
  Files          41       41           
  Lines        3101     3103    +2     
  Branches      671      670    -1     
=======================================
+ Hits         3075     3077    +2     
  Misses         14       14           
  Partials       12       12           

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[DanielNoord]

Please remove this, we don't keep the changes here

[DanielNoord]

Why can't we use strip() here?