Fix STRIP_ONE trailing newline handling

[oiahoon]

Summary

• make strip_pre=STRIP_ONE anchor its trailing-newline match at the true end of the input
• add regression coverage for two and three trailing bare newlines
• preserve the existing behavior for space-padded blank lines

Fixes #255.

Verification

• pytest tests/test_args.py -q
• pytest -q
• flake8 --ignore=E501,W503 markdownify tests
• restructuredtext-lint README.rst
• mypy .
• mypy --strict tests/types.py
• python -m build -nwsx .
• git diff --check