codecs.open() is deprecated

[mtelka]

Testing with Python 3.14.0 shows this warning:

gitdb/test/db/test_ref.py::TestReferenceDB::test_writing
gitdb/test/db/test_ref.py::TestReferenceDB::test_writing
gitdb/test/db/test_ref.py::TestReferenceDB::test_writing
gitdb/test/db/test_ref.py::TestReferenceDB::test_writing
  $(@D)/gitdb/db/ref.py:45: DeprecationWarning: codecs.open() is deprecated. Use open() instead.
    with codecs.open(self._ref_file, 'r', encoding="utf-8") as f:

[Byron]

Thanks a lot. If using open in its place is backwards compatible within the v3 version of python, this should be an easy fix.