Use AppVeyor's built-in pip

Author: dahlia

appveyor.yml

@@ -25,8 +25,6 @@ init:
 - ps: "ls C:/Python*"
 install:
 - "git -C %APPVEYOR_BUILD_FOLDER% submodule update --init"
-- ps: (new-object net.webclient).DownloadFile('https://bootstrap.pypa.io/get-pip.py', 'C:/get-pip.py')
-- "%PYTHON%\\python.exe C:/get-pip.py"
 - "%PYTHON%\\Scripts\\pip.exe --version"
 - "%PYTHON%\\Scripts\\pip.exe install wheel"
 - "%PYTHON%\\Scripts\\pip.exe install -e ."