Explicitly set language_level to 3

[dkropachev]

Fix and get rid of following warnings:

    C:\Users\runneradmin\AppData\Local\Temp\cibw-run-7_hzrlyd\cp38-win32\venv-test\lib\site-packages\Cython\Compiler\Main.py:369: FutureWarning: Cython directive 'language_level' not set, using 2 for now (Py2). This will change in a later release! File: D:\a\python-driver\python-driver\tests\unit\cython\bytesio_testhelper.pyx