CI: Use pypiwin32 over pywin32

Author: effigies

appveyor.yml

@@ -21,7 +21,7 @@ install:
   # the parent CMD process).
   - SET PATH=%PYTHON%;%PYTHON%\Scripts;%PATH%
 
-  - python -m pip install --upgrade pip virtualenv pywin32
+  - python -m pip install --upgrade pip virtualenv pypiwin32
   - virtualenv --python=python venv
   - venv\Scripts\activate
   - python --version