Use intersphinx for links to the tox documentation

docs/conf.py

@@ -248,7 +248,10 @@
 
 
 # Example configuration for intersphinx: refer to the Python standard library.
-intersphinx_mapping = {'http://docs.python.org/': None}
+intersphinx_mapping = {
+    'python': ('http://docs.python.org/3', None),
+    'tox': ('https://tox.readthedocs.io/en/latest', None),
+}
 
 
 # --- Nitpicking options ------------------------------------------------------

docs/index.rst

@@ -42,12 +42,11 @@ Tests
 `pytest`_ is the preferred way to run tests. Just run: ``pytest`` from the
 root directory to execute tests using the default Python interpreter.
 
-`tox`_ could be used to run tests for all supported Python versions.
-Install it (using 'pip install tox') and then run ``tox`` from
+:doc:`tox <tox:index>` could be used to run tests for all supported Python
+versions. Install it (using 'pip install tox') and then run ``tox`` from
 the root directory - tests will be executed for all available
 Python interpreters.
 
-.. _tox: http://tox.testrun.org
 .. _pytest: https://docs.pytest.org/en/latest/