Drop Python 2 support since support reached the end

[Ivan Kochurkin]

The following warning produced if pip 2 is started:

DEPRECATION: Python 2.7 reached the end of its life on January 1st, 2020. Please upgrade your Python as Python 2.7 is no longer maintained. pip 21.0 will drop support for Python 2.7 in January 2021. More details about Python 2 support in pip, can be found at https://pip.pypa.io/en/latest/development/release-process/#python-2-support

Also, it makes the support of two versions of Python more complicated.