fix: do not add a line continuation to a file without a trailing newline

[Eljees]

docformatter --in-place appends a backslash to the last line of any file that
does not end with a newline, which leaves the file unparseable:

$ printf 'def foo():\n    """\n    Hello foo.\n    """\n    x = 1' > a.py   # no trailing newline
$ docformatter --in-place a.py
$ tail -1 a.py
    x = 1\
$ python -c "import ast; ast.parse(open('a.py').read())"
SyntaxError: unexpected EOF while parsing

This is a regression from #360 (4849fce), found by bisecting the test suite. That
commit changed is_newline_continuation() in classify.py from

and token.line.strip() in prev_token.line.strip()
and token.line not in {"\n", "\r\n"}

to

and token.line.strip()
and token.line.strip() in prev_token.line.strip()

The intent was to stop treating a blank line with trailing whitespace as a
continuation (#355). token.line.strip() also drops the synthetic NEWLINE token
that tokenize emits for a source without a trailing newline -- that token has
line=''. It stops counting as a continuation, _get_unmatched_start_end_indices
moves it onto the next row, and untokenize bridges the gap with a backslash.

The fix adds a small _is_blank_line() helper -- a line that is non-empty and all
whitespace -- so '' passes again while " \n" is still excluded. #355 stays
fixed; I kept its reproducer as a control and it is still green.

Only 3.10 and 3.11 are affected. Under PEP 701 the 3.12+ tokenizer reports the real
line for the synthetic NEWLINE, so the old branch keeps working:

interpreter	tokenize for x = 1 without \n	test_do_format_code.py
3.10.12, 3.10.20	NEWLINE '' line=''	10 failed, 61 passed
3.12.13	NEWLINE '' line='x = 1'	71 passed

pyproject.toml declares python = "^3.10", so this is a supported version.

Why CI is green

The version matrix does not run the versions it names. From the log of job
ubuntu-latest Python: 3.10, run 30781552231:

tox-gh-actions config: {'python': {'3.10': ['py310'], '3.11': ['py311'], ...}}
using the following factors to decide envlist: ['py313']
platform linux -- Python 3.13.14, pytest-9.1.1

tox is installed under setup-python 3.13, and tox -e py --skip-pkg-install
takes basepython from tox itself, so all six matrix jobs run 3.13.14. That is why
this survived on master for three weeks. Happy to send a separate PR for the
workflow if you want it -- I left it out of this one.

Verification

On e154acb, Python 3.10.12: master is 10 failed, 536 passed; with this change
546 passed, 4 skipped, plus the test added here. Reverting the code and keeping
the test turns exactly issue_360_no_trailing_newline red, with the diff
x = 1 -> x = 1\, while issue_355 stays green.