[travis] correct binary for windows

.travis.yml

@@ -8,17 +8,17 @@ jobs:
       os: linux
       dist: bionic
       before_install: sudo apt-get -y install python3-pip python3-setuptools
-      env: SETUPPREFIX="--user"
+      env: PY=python3
     - name: "macOS"
       os: osx
       before_install: python3 --version; pip3 --version; sw_vers
-      env: SETUPPREFIX=""
+      env: PY=python3
     - name: "Windows"
       os: windows
       before_install:
         - choco install python --version 3.8.0
         - python -m pip install --upgrade pip
-      env: PATH=/c/Python38:/c/Python38/Scripts:$PATH SETUPPREFIX=""
+      env: PATH=/c/Python38:/c/Python38/Scripts:$PATH PY=python
   allow_failures:
     - os: linux
     - os: windows
@@ -30,6 +30,6 @@ before_script:
 
 script:
     - pip3 install numpy pandas
-    - python3 setup.py install $SETUPPREFIX
-    - PATH=$HOME/.local/bin:$PATH python3 test/test.py
+    - $PY setup.py install $SETUPPREFIX
+    - PATH=$HOME/.local/bin:$PATH $PY test/test.py