appveyor: Install pywin32 extensions.
AppVeyor build images seem to have been upgraded to Python 2.7.12, but
no longer have pywin32 pre-installed.
diff --git a/appveyor.yml b/appveyor.yml
index 2e9b9d6..2618f69 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -47,6 +47,8 @@
- python -m pip --version
# Install Mako
- python -m pip install --egg Mako
+# Install pywin32 extensions, needed by SCons
+- python -m pip install pypiwin32
# Install SCons
- python -m pip install --egg scons==2.4.1
- scons --version