Hint Python 3.7 path on AppVeyor.

.appveyor.yml

@@ -11,6 +11,7 @@ environment:
   matrix:
   - config: Debug
     BOOST_ROOT: C:/Libraries/boost_1_65_1
+    PYTHON_VERSION: 37
 install:
   - cinstall: python
 build:
@@ -34,6 +35,7 @@ build_script:
   # - ${PYTHON} 101_FileIO.py
   # - cd ../../
   # Tutorials and tests
+  - SET PATH=C:\Python%PYTHON_VERSION%_64;%PATH%
   - mkdir build
   - cd build
   - cmake -DCMAKE_BUILD_TYPE=%config% -DLIBIGL_WITH_CGAL=ON -DLIBIGL_WITH_COMISO=OFF -G "Visual Studio 15 2017 Win64" ../