use python3

Author: masci

appveyor.yml

@@ -21,7 +21,7 @@ install:
   - go get github.com/golangci/govet
   - go get golang.org/x/lint/golint
   # Python dependencies needed at test time
-  - pip install -r test\requirements.txt
+  - pip3 install -r test\requirements.txt
   # protobuf tooling needed at test time
   - go get github.com/golang/protobuf/protoc-gen-go
   - curl -o protoc.zip -LO https://github.com/protocolbuffers/protobuf/releases/download/v3.8.0/protoc-3.8.0-win64.zip