Set the cloned directory as PYTHONPATH in appveyor.yml
diff --git a/appveyor.yml b/appveyor.yml
index 93cfd469e55..7fd63686499 100644
--- a/appveyor.yml
+++ b/appveyor.yml
@@ -12,7 +12,8 @@ branches:
 
 install:
   - "SET PATH=%PYTHON%;%PYTHON%\\Scripts;%PATH%"
-  - "SET TOX_TESTENV_PASSENV=HOME USERPROFILE HOMEPATH HOMEDRIVE"
+  - "SET PYTHONPATH=%APPVEYOR_BUILD_FOLDER%"
+  - "SET TOX_TESTENV_PASSENV=HOME HOMEDRIVE HOMEPATH PYTHONPATH USERPROFILE"
   - "pip install -U tox"
 
 build: false

Fixes #3809

I’m unsure how it broke, I suspect it was a change in the AppVeyor side.