Fix: Appveyor errors related to missing python2

https://ci.appveyor.com/project/vedang/pdf-tools/builds/38649076
failed with `python2 not found`, which was the proposed fix in
politza/pdf-tools@628 . My guess is that `python2` now needs to be
explicitly installed in mingw64.

This commit adds an explicit dependency on `mingw-w64-x86_64-python2`
to test this.

Closes politza/pdf-tools@628

ci/appveyor/install.bat

@@ -20,7 +20,7 @@ IF %COMPILER%==msys2 (
   bash -lc "pacman -S --needed --noconfirm git"
 
   REM dependencies
-  bash -lc "pacman -S --needed --noconfirm mingw-w64-x86_64-zlib mingw-w64-x86_64-libpng mingw-w64-x86_64-poppler mingw-w64-x86_64-imagemagick openssl mingw-w64-x86_64-openssl"
+  bash -lc "pacman -S --needed --noconfirm mingw-w64-x86_64-zlib mingw-w64-x86_64-libpng mingw-w64-x86_64-poppler mingw-w64-x86_64-imagemagick openssl mingw-w64-x86_64-openssl mingw-w64-x86_64-python2"
 
   REM Set up emacs
   bash -lc "pacman -S --needed --noconfirm mingw-w64-x86_64-emacs"