tox anypy environment

Author: nedbat

tox.ini

@@ -50,9 +50,9 @@ commands =
 [testenv:py39]
 basepython = python3.9
 
-[testenv:tip]
-# For running against the tip of master of CPython.
-basepython = /usr/local/cpython/bin/python3.9
+[testenv:anypy]
+# For running against my own builds of CPython, or any other specific Python.
+basepython = {env:COVERAGE_PYTHON}
 
 [testenv:doc]
 # Build the docs so we know if they are successful.  We build twice: once with