tox/appveyor: try creating pythonX.Y.bat file to run with specific python.exe

https://nedbatchelder.com/blog/201509/appveyor.html

The old TOXPYTHON trick seems to be discouraged now with tox 3.1, which
emits a warning when there is a conflict in the basepython settings for
environments containing default factors (e.g. py27, etc.)
https://github.com/tox-dev/tox/pull/841

appveyor.yml

@@ -5,13 +5,11 @@ environment:
       PYTHON_VERSION: "2.7.x"
       PYTHON_ARCH: "32"
       TOXENV: "py27-cov,py27-cov-lxml"
-      TOXPYTHON: "C:\\Python27\\python.exe"
 
     - PYTHON: "C:\\Python36-x64"
       PYTHON_VERSION: "3.6.x"
       PYTHON_ARCH: "64"
       TOXENV: "py36-cov,py36-cov-lxml"
-      TOXPYTHON: "C:\\Python36-x64\\python.exe"
 
     - PYTHON: "C:\\Python37-x64"
       PYTHON_VERSION: "3.7.x"
@@ -19,7 +17,6 @@ environment:
       # lxml doesn't have windows 3.7 wheels yet
       # TOXENV: "py37-cov,py37-cov-lxml"
       TOXENV: "py37-cov"
-      TOXPYTHON: "C:\\Python37-x64\\python.exe"
 
 skip_branch_with_pr: true
 
@@ -50,6 +47,12 @@ install:
   # install tox to run test suite in a virtual environment
   - "pip install -U tox"
 
+  # Make a python3.7.bat file in the current directory so that tox will find it
+  # and python3.7 will mean what we want it to. E.g. for TOXENV=py27, this will
+  # save a python2.7.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:]), 'w').write('@{0}\\\\python \\x25*\\n'.format(os.environ['PYTHON']))\""
+
 build: false
 
 test_script:

tox.ini

@@ -4,12 +4,6 @@ minversion = 2.9.1
 skip_missing_interpreters = true
 
 [testenv]
-basepython =
-    # we use TOXPYTHON env variable to specify the location of Appveyor Python
-    py27: {env:TOXPYTHON:python2.7}
-    py35: {env:TOXPYTHON:python3.5}
-    py36: {env:TOXPYTHON:python3.6}
-    py37: {env:TOXPYTHON:python3.7}
 description = run the tests with pytest under {basepython}
 setenv =
   COVERAGE_FILE={toxinidir}/.coverage.{envname}
@@ -24,7 +18,6 @@ commands =
 [testenv:coverage]
 description = run locally after tests to combine coverage data and create reports;
               generates a diff coverage against origin/master (or DIFF_AGAINST env var)
-basepython = {env:TOXPYTHON:python}
 deps =
   coverage >= 4.4.1, < 5
   diff_cover
@@ -44,7 +37,6 @@ commands =
 
 [testenv:codecov]
 description = upload coverage data to codecov (only run on CI)
-basepython = {env:TOXPYTHON:python}
 deps =
   {[testenv:coverage]deps}
   codecov
@@ -58,7 +50,6 @@ commands =
 
 [testenv:sdist]
 description = build sdist to be uploaded to PyPI
-basepython = {env:TOXPYTHON:python}
 skip_install = true
 deps =
   setuptools >= 36.4.0
@@ -70,7 +61,6 @@ commands =
 
 [testenv:wheel]
 description = build wheel package for upload to PyPI
-basepython = {env:TOXPYTHON:python}
 skip_install = true
 deps = {[testenv:sdist]deps}
 changedir = {toxinidir}