setup: clarify Python versions

We run tests with Python 3.6 and newer. Python 3.5 has reached its end
of life two years ago [1]. Our code already doesn't work on
Python 3.5 [2].

1. https://endoflife.date/python
2. https://github.com/tarantool/tarantool-python/actions/runs/3243224481/jobs/5317632662

Part of #232

Author: DifferentialOrange

setup.py

@@ -85,5 +85,5 @@ def find_version(*file_paths):
     install_requires=[
         'msgpack>=1.0.4',
     ],
-    python_requires='>=3',
+    python_requires='>=3.6',
 )