Update zstandard dependency and add Python 3.14 fix

Updated the zstandard dependency to version 0.23.1 in README.md and pyproject.toml. Added a compatibility fix in src/tzst/core.py for Python 3.14 due to the removal of ByteString from the typing module.

pyproject.toml

@@ -23,7 +23,7 @@ classifiers = [
     "Topic :: System :: Archiving :: Compression",
     "Topic :: Software Development :: Libraries :: Python Modules",
 ]
-dependencies = ["zstandard>=0.23.0,<1.0.0"]
+dependencies = ["zstandard>=0.23.1,<1.0.0"]
 
 [project.optional-dependencies]
 dev = ["pytest>=7.0.0", "pytest-cov>=4.0.0", "ruff>=0.1.0", "black>=23.0.0"]