Document minimum python version

[bmispelon]

Hi,

I was trying to use this plugin for a Python 3.7 project and ran into an issue because of the use of functools.cached_property (which is new in 3.8).

Here is a PR that adds a small note in the install section of the README, plus a setting in setup.py to prevent installing the plugin on incompatible python versions (see https://packaging.python.org/guides/distributing-packages-using-setuptools/#python-requires).

A better fix might be to remove the dependency on cached_property but that might require more effort whereas this seems like a quick and easy fix.

Thanks ✨

[bmispelon]

I did a bit of digging around and from what I understand the config attribute is set when Visitor is instanciated: https://github.com/afonasev/flake8-plugin-utils/blob/master/flake8_plugin_utils/plugin.py#L42

And the Visitor is instanciated only once when the plugin is run: https://github.com/afonasev/flake8-plugin-utils/blob/master/flake8_plugin_utils/plugin.py#L70

This means that replacing cached_property with a plain property shouldn't be too expensive performance-wise since we're only accessing existing attributes, not doing any extra computation.