Set a minimal version of Python to work with

[arxitim]

Good day!I really liked your library and wanted to add it to my CI.
But it completely slipped my mind that I'm working with a pretty old version of Python3.5

It was easy to install the package and I already started adding it to my master, but after that no CI was successful.
This is because this package uses some features of newer versions of Python that aren't available in 3.5, like types in variable declaration.
Unfortunately, I didn't realize this right away and wasted a lot of time researching it.

I suggest adding the python_requires directive to setup.py/setup.cfg so that I and other users of older versions of Python run into the problem already at the installation stage of the package

I understand that this is stated in both the project badges and the description, but sometimes we all make mistakes, and this clarification could save a little time)