azure-pipelines: Fix cygwin python3 installation

The package has been moved from python3-pip to python3N-pip where N is
5, 6, 7. We use Python 3.5, so let's use that.

azure-pipelines.yml

@@ -76,13 +76,13 @@ jobs:
         libglib2.0-devel,^
         libgtk3-devel,^
         ninja,^
-        python3-pip,^
+        python35-pip,^
         vala,^
         zlib-devel
       displayName: Install Dependencies
     - script: |
         set PATH=%CYGWIN_ROOT%\bin;%SYSTEMROOT%\system32
-        env.exe -- python3 run_tests.py --backend=ninja
+        env.exe -- python3.5 run_tests.py --backend=ninja
       displayName: Run Tests
     - task: CopyFiles@2
       condition: not(canceled())