Revert "Remove an ugly way to pass python version to tox"

This reverts commit 960ca66.
The reason why it's needed is that in tox, we write conditions like
[testenv:py38], but this does not apply to "tox -e py"
as the environment name is different.
I wonder what is the proper way to do this?

Author: lopuhin

.github/workflows/python-package.yml

@@ -30,4 +30,4 @@ jobs:
         python -m pip install -r requirements-dev.txt
     - name: tox
       run: |
-        tox -e py
+        tox -e `python -c "import sys; print('py' + ''.join(sys.version.split('.')[:2]))"`