Replace codecs.open with a simplified reimplementation

python 3.14 doesn't like codecs.open:

tests/test_readers.py: 1 warning
/builddir/build/BUILD/diffoscope-303-build/diffoscope-303/diffoscope/comparators/text.py:44: DeprecationWarning: codecs.open() is deprecated. Use open() instead.
   ) as my_content, codecs.open(
...

In most scenarios, codecs.open and builtins.open are equivalent.
Unfortunately, the case where the file is opened for reading, 'b' is not
used in the mode, and an encoding is specified, results in a difference
in behaviour: whitespace is handled differently.

To avoid the warning, add a trivial reimplementation.