tox: don't set basepython to %TOXPYTHON% on Windows

We can simply call 'tox -e py' which means create a tox env with the current
'python' interpreter in the $PATH.
This also allows one to call tox with a python version that is not explicitly
listed in the [testenv] basepython list.
For example, I needed to run tests on python3.5 locally and I couldn't unless
I explicitly listed it in there.
That TOXPYTHON thing was a hack, this is the recommended way to use tox on
windows.

.appveyor.yml

@@ -5,43 +5,33 @@ environment:
   global:
     TWINE_USERNAME: "adobe-type-tools-ci"
     # TWINE_PASSWORD is set in Appveyor settings
+    TOXENV: "py-cov"
 
   matrix:
     - PYTHON: "C:\\Python27"
       PYTHON_VERSION: "2.7"
       PYTHON_ARCH: "32"
-      TOXENV: "py27-cov-tx"
-      TOXPYTHON: "C:\\Python27\\python.exe"
+      TOXENV: "py-cov-tx"
 
     # - PYTHON: "C:\\Python36"
     #   PYTHON_VERSION: "3.6"
     #   PYTHON_ARCH: "32"
-    #   TOXENV: "py36-cov"
-    #   TOXPYTHON: "C:\\Python36\\python.exe"
 
     # - PYTHON: "C:\\Python37"
     #   PYTHON_VERSION: "3.7"
     #   PYTHON_ARCH: "32"
-    #   TOXENV: "py37-cov"
-    #   TOXPYTHON: "C:\\Python37\\python.exe"
 
     # - PYTHON: "C:\\Python27-x64"
     #   PYTHON_VERSION: "2.7"
     #   PYTHON_ARCH: "64"
-    #   TOXENV: "py27-cov"
-    #   TOXPYTHON: "C:\\Python27-x64\\python.exe"
 
     - PYTHON: "C:\\Python36-x64"
       PYTHON_VERSION: "3.6"
       PYTHON_ARCH: "64"
-      TOXENV: "py36-cov"
-      TOXPYTHON: "C:\\Python36-x64\\python.exe"
 
     - PYTHON: "C:\\Python37-x64"
       PYTHON_VERSION: "3.7"
       PYTHON_ARCH: "64"
-      TOXENV: "py37-cov"
-      TOXPYTHON: "C:\\Python37-x64\\python.exe"
 
 matrix:
   fast_finish: true

tox.ini

@@ -12,10 +12,6 @@ addopts =
   --exitfirst
 
 [testenv]
-basepython =
-    py27: {env:TOXPYTHON:python2.7}
-    py36: {env:TOXPYTHON:python3.6}
-    py37: {env:TOXPYTHON:python3.7}
 description = run the tests with pytest under {basepython}
 setenv =
   COVERAGE_FILE={toxinidir}/.coverage.{envname}
@@ -30,7 +26,6 @@ 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
@@ -50,7 +45,6 @@ commands =
 
 [testenv:coverage-c]
 description = generate coverage for C library
-basepython = {env:TOXPYTHON:python}
 deps =
   -rrequirements.txt
   pytest-xdist
@@ -68,7 +62,6 @@ commands =
 
 [testenv:codecov]
 description = upload Python coverage data to codecov (only run on CI)
-basepython = {env:TOXPYTHON:python}
 deps =
   {[testenv:coverage]deps}
   codecov
@@ -82,7 +75,6 @@ commands =
 
 [testenv:coverage-codecov-c]
 description = generate C coverage data and upload to codecov (only run on CI)
-basepython = {env:TOXPYTHON:python}
 deps =
   {[testenv:coverage-c]deps}
   codecov
@@ -96,7 +88,6 @@ commands =
 
 [testenv:sdist]
 description = build sdist to be uploaded to PyPI
-basepython = {env:TOXPYTHON:python}
 skip_install = true
 deps =
   setuptools >= 36.4.0
@@ -108,7 +99,6 @@ commands =
 
 [testenv:wheel]
 description = build wheel package for upload to PyPI
-basepython = {env:TOXPYTHON:python}
 skip_install = true
 deps = {[testenv:sdist]deps}
 changedir = {toxinidir}