Add setuptools_scm to the host environment

Author: nicoddemus

appveyor.yml

@@ -73,7 +73,7 @@ install:
   # compiled extensions and are not provided as pre-built wheel packages,
   # pip will build them from source using the MSVC compiler matching the
   # target Python version and architecture
-  - "%CMD_IN_ENV% pip install tox"
+  - "%CMD_IN_ENV% pip install tox setuptools_scm"
 
 build: false  # Not a C# project, build stuff at the test step instead.