According to the docs, "/Od" is the default in MSVC, so in order to get it, it...

According to the docs, "/Od" is the default in MSVC, so in order to get it, it should be enough to clear CFLAGS.
Setting it generates warnings in Py3.9 on appveyor ("source file '"/Od"' ignored"), suggesting that the option is not recognised as such.
https://docs.microsoft.com/en-us/cpp/build/reference/od-disable-debug?view=msvc-160

appveyor.yml

@@ -90,7 +90,7 @@ build_script:
 test: off
 test_script:
   - "%PYTHON%\\Scripts\\pip.exe install -r test-requirements.txt"
-  - "set CFLAGS=/Od"
+  - "set CFLAGS="
   - "%WITH_ENV% %PYTHON%\\python.exe runtests.py -vv --no-cpp --no-code-style -j5"
 
 artifacts: