Fixed the build process compatibility with Python 3.8.

[nuald]

Addressed the RuntimeError during the make command (please see https://docs.python.org/3.9/whatsnew/3.8.html#changes-in-the-python-api):

A RuntimeError is now raised when the custom metaclass doesn’t provide the __classcell__ entry in the namespace passed to type.__new__. A DeprecationWarning was emitted in Python 3.6–3.7.