fix: decode external data file content explicitly as UTF-8

[sisp]

I've fixed a string encoding/decoding bug on Windows that broke loading external data with non-ASCII characters. See #1970 for more details, as the root cause for this problem is the same.

I've verified that the new test is failing on Windows 11 without the fix:

Expand to view the pytest log

$  poetry run pytest tests/test_answersfile.py::test_external_data_with_umlaut -n1
=============================================================================== test session starts ================================================================================
platform win32 -- Python 3.13.2, pytest-8.3.4, pluggy-1.5.0
rootdir: C:\Users\<REDACTED>\Documents\copier
configfile: pyproject.toml
plugins: cov-6.0.0, gitconfig-0.7.0, xdist-3.6.1
1 worker [1 item]
F                                                                                                                                                                             [100%]
===================================================================================== FAILURES =====================================================================================
__________________________________________________________________________ test_external_data_with_umlaut __________________________________________________________________________
[gw0] win32 -- Python 3.13.2 C:\Users\<REDACTED>\AppData\Local\pypoetry\Cache\virtualenvs\copier-mBQJSSRS-py3.13\Scripts\python.exe

tmp_path_factory = TempPathFactory(_given_basetemp=WindowsPath('C:/Users/local_<REDACTED>/Temp/pytest-of-<REDACTED>/pytest-5/popen-gw0'), _tr...ath('C:/Users/local_<REDACTED>/Temp/pytest-of-<REDACTED>/pytest-5/popen-gw0'), _retention_count=3, _retention_policy='all')

    def test_external_data_with_umlaut(
        tmp_path_factory: pytest.TempPathFactory,
    ) -> None:
        src, dst = map(tmp_path_factory.mktemp, ("src", "dst"))

        build_file_tree(
            {
                (src / "copier.yml"): (
                    """\
                    _external_data:
                        data: data.yml
                    ext_umlaut: "{{ _external_data.data.umlaut }}"
                    """
                ),
                (src / "{{ _copier_conf.answers_file }}.jinja"): (
                    "{{ _copier_answers|to_nice_yaml }}"
                ),
            }
        )
        git_save(src, tag="v1")

        (dst / "data.yml").write_text("umlaut: äöü", encoding="utf-8")

        copier.run_copy(str(src), dst, defaults=True, overwrite=True)
        answers = load_answersfile_data(dst, ".copier-answers.yml")
>       assert answers["ext_umlaut"] == "äöü"
E       AssertionError: assert 'äöü' == 'äöü'
E
E         - äöü
E         + äöü

tests\test_answersfile.py:252: AssertionError
------------------------------------------------------------------------------- Captured stdout call -------------------------------------------------------------------------------


------------------------------------------------------------------------------- Captured stderr call -------------------------------------------------------------------------------

Copying from template version 1
    create  .copier-answers.yml
============================================================================= short test summary info ==============================================================================
FAILED tests/test_answersfile.py::test_external_data_with_umlaut - AssertionError: assert 'äöü' == 'äöü'
================================================================================ 1 failed in 5.55s =================================================================================

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.90%. Comparing base (21b486c) to head (0ff1e82).
Report is 125 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1973   +/-   ##
=======================================
  Coverage   97.89%   97.90%           
=======================================
  Files          53       53           
  Lines        5567     5575    +8     
=======================================
+ Hits         5450     5458    +8     
  Misses        117      117           

Flag	Coverage Δ
unittests	97.90% <100.00%> (+<0.01%)	⬆️

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