So that set_env can show me one more variable

Author: nedbat

tox.ini

@@ -48,6 +48,7 @@ commands =
     python igor.py test_with_tracer c {posargs}
 
 [testenv:anypy]
+# $set_env.py: COVERAGE_PYTHON - The custom Python for "tox -e anypy"
 # For running against my own builds of CPython, or any other specific Python.
 basepython = {env:COVERAGE_PYTHON}