tox/appveyor: don't use TOXPYTHON env var to select basepython; write a pythonX.Y.bat wrapper instead

.appveyor.yml

@@ -3,17 +3,14 @@ environment:
     - JOB: "2.7 32-bit"
       PYTHON_HOME: "C:\\Python27"
       TOXENV: "py27-cov"
-      TOXPYTHON: "C:\\Python27\\python.exe"
 
     - JOB: "3.6 64-bit"
       PYTHON_HOME: "C:\\Python36-x64"
       TOXENV: "py36-cov"
-      TOXPYTHON: "C:\\Python36-x64\\python.exe"
 
     - JOB: "3.7 64-bit"
       PYTHON_HOME: "C:\\Python37-x64"
       TOXENV: "py37-cov"
-      TOXPYTHON: "C:\\Python37-x64\\python.exe"
 
 install:
   # If there is a newer build queued for the same PR, cancel this one.
@@ -39,6 +36,12 @@ install:
   # install the dependencies to run the tests
   - "python -m pip install tox"
 
+  # Make a 'pythonX.X.bat' file in the current directory so that tox will find
+  # it and 'pythonX.X' will mean what we want it to. E.g. for 'TOXENV=py37',
+  # this will save a 'python3.7.bat' file containing "@C:\Python27\python %*"
+  # Credit: https://nedbatchelder.com/blog/201509/appveyor.html
+  - "python -c \"import os; open('python{0}.{1}.bat'.format(*os.environ['TOXENV'][2:4]), 'w').write('@{0}\\\\python \\x25*\\n'.format(os.environ['PYTHON']))\""
+
 
 build: false
 

tox.ini

@@ -2,13 +2,6 @@
 envlist = py{27,36,37}-cov, htmlcov
 
 [testenv]
-basepython =
-    py27: {env:TOXPYTHON:python2.7}
-    pypy: {env:TOXPYTHON:pypy}
-    py34: {env:TOXPYTHON:python3.4}
-    py35: {env:TOXPYTHON:python3.5}
-    py36: {env:TOXPYTHON:python3.6}
-    py37: {env:TOXPYTHON:python3.7}
 deps =
     cov: coverage>=4.3
     pytest
@@ -24,7 +17,6 @@ commands =
     nocov: pytest {posargs}
 
 [testenv:htmlcov]
-basepython = {env:TOXPYTHON:python3.6}
 deps =
     coverage>=4.3
 skip_install = true
@@ -34,7 +26,6 @@ commands =
 
 [testenv:codecov]
 passenv = *
-basepython = {env:TOXPYTHON:python}
 deps =
     coverage>=4.3
     codecov
@@ -45,7 +36,6 @@ commands =
     codecov --env TOXENV
 
 [testenv:bdist]
-basepython = {env:TOXPYTHON:python3.6}
 deps =
     pygments
     docutils