Replace open instead of codecs.open in simple cases

python 3.14 doesn't like codecs.open:

tests/test_readers.py: 1 warning
tests/comparators/test_gettext.py::test_charsets
  /builddir/build/BUILD/diffoscope-303-build/diffoscope-303/tests/comparators/test_gettext.py:61: DeprecationWarning: codecs.open() is deprecated. Use open() instead.
    expected_diff = codecs.open(
tests/comparators/test_text.py::test_difference_in_unicode
  /builddir/build/BUILD/diffoscope-303-build/diffoscope-303/tests/comparators/test_text.py:53: DeprecationWarning: codecs.open() is deprecated. Use open() instead.
    expected_diff = codecs.open(
tests/comparators/test_text.py::test_difference_between_iso88591_and_unicode
  /builddir/build/BUILD/diffoscope-303-build/diffoscope-303/tests/comparators/test_text.py:64: DeprecationWarning: codecs.open() is deprecated. Use open() instead.
    expected_diff = codecs.open(
tests/comparators/test_text.py::test_difference_between_iso88591_and_unicode_only
  /builddir/build/BUILD/diffoscope-303-build/diffoscope-303/tests/comparators/test_text.py:74: DeprecationWarning: codecs.open() is deprecated. Use open() instead.
    codecs.open(data("text_iso8859"), encoding="iso8859-1")
tests/comparators/test_utils.py::test_fuzzy_matching
  /builddir/build/BUILD/diffoscope-303-build/diffoscope-303/tests/comparators/test_utils.py:75: DeprecationWarning: codecs.open() is deprecated. Use open() instead.
    expected_diff = codecs.open(
...

When reading text files and when writing files, plain open() is
equivalent.