tox: split out default system python target

Since newer tox versions drop implicit `tox -e python` functionality.

Author: radhermit

tox.ini

@@ -12,7 +12,6 @@ setenv =
     PYTEST_ADDOPTS = -p no:pkgcore
 
 [testenv]
-description = run tests for python version
 skip_sdist = true
 skip_install = true
 passenv =
@@ -28,6 +27,9 @@ deps =
     pytest
     pytest-cython
     tomli ; python_version < '3.11'
+
+[testenv:python]
+description = run tests for default system python
 commands =
     python setup.py build_ext --inplace --develop
     # run doctests