Add deprecation warning for python 3.8

[Kamforka]

Python 3.8 is already end of life, include a deprecation warning in the main module about dropping support for it in the near future.

Something like:

import sys
import warnings

from thehive4py.client import TheHiveApi

if sys.version_info < (3, 9):
    warnings.warn(
        message=(
            "The `thehive4py` library will soon drop support for Python "
            "versions below 3.9, as they have reached their end of life. "
            "Please upgrade to a newer version as soon as possible."
        ),
        category=DeprecationWarning,
        stacklevel=2,
    )