Add pypy to install_python.ps1

[mattip]

It turns out opencv-python (the repo to build wheels for OpenCV) uses multibuild, and depends on install_python.ps1 to install python on appveyor. It would be nice if install_python.ps1 would allow PYTHON-="C:\\Pypy37" as an option, which would

• download from https://downloads.python.org/pypy/
• unzip the file into C:\\PyPy37
• call C:\\PyPy37\pypy3.exe -m ensurepip