[appveyor] Exclude tests due to SSL fails

The tests test_handle_include and test_handle_repository
from the TestVersionConverter class consistently fail with
an SSL certification failure on the appveyor Windows builds.
Upgrading the "certifi" package does not resolve this issue.

Since these tests run fine on Linux, Mac and on the GH actions
Windows builds, these tests are deactivated for Appveyor builds.

appveyor.yml

@@ -45,4 +45,7 @@ install:
 
 test_script:
   - python --version
-  - python -m pytest -v
+  - python -m pytest -v -k "not handle_include and not handle_repository"
+# appveyor issues with SSL certificates; deactivating the affected tests,
+# since they run fine on GH actions Windows builds.
+# - python -m pytest -v