RFE: use `zoneinfo` instead of `pytz`

[kloczek]

Switch to standard zoneinfo module.
Below may help pydantic/pydantic-core@fd262933

[tkloczko@pers-jacek pyiso8601-2.1.0]$ grep -r pytz
iso8601/test_iso8601.py:import hypothesis.extra.pytz
iso8601/test_iso8601.py:    s=hypothesis.strategies.datetimes(timezones=hypothesis.extra.pytz.timezones())
noxfile.py:    session.install("pytest", "hypothesis", "pytz", "black", "mypy", "ruff")
noxfile.py:    session.install("pytest", "hypothesis", "pytz")
poetry.lock:pytz = {version = ">=2015.7", markers = "python_version < \"3.9\""}
poetry.lock:all = ["backports.zoneinfo (>=0.2.1)", "black (>=19.10b0)", "click (>=7.0)", "django (>=3.2)", "dpcontracts (>=0.4)", "importlib-metadata (>=3.6)", "lark (>=0.10.1)", "libcst (>=0.3.16)", "numpy (>=1.17.3)", "pandas (>=1.1)", "pytest (>=4.6)", "python-dateutil (>=1.4)", "pytz (>=2014.1)", "redis (>=3.0.0)", "rich (>=9.0.0)", "tzdata (>=2023.3)"]
poetry.lock:pytz = ["pytz (>=2014.1)"]
poetry.lock:name = "pytz"
poetry.lock:    {file = "pytz-2023.3.post1-py2.py3-none-any.whl", hash = "sha256:ce42d816b81b68506614c11e8937d3aa9e41007ceb50bfdcb0749b921bf646c7"},
poetry.lock:    {file = "pytz-2023.3.post1.tar.gz", hash = "sha256:7b4fddbeb94a1eba4b557da24f19fdf9db575192544270a9101d8509f9f43d7b"},
pyproject.toml:pytz = "*"