vagrant: use python 3.9.4

3.9.3 was withdrawn by python.org and pyenv devs.

Vagrantfile

@@ -161,7 +161,7 @@ def install_pythons(boxname)
     pyenv install 3.12.0  # tests
     pyenv install 3.11.7  # tests, binary build
     pyenv install 3.10.2  # tests
-    pyenv install 3.9.3  # tests
+    pyenv install 3.9.4  # tests
     pyenv rehash
   EOF
 end
@@ -226,8 +226,8 @@ def run_tests(boxname, skip_env)
     . ../borg-env/bin/activate
     if which pyenv 2> /dev/null; then
       # for testing, use the earliest point releases of the supported python versions:
-      pyenv global 3.9.3 3.10.2 3.11.7 3.12.0
-      pyenv local 3.9.3 3.10.2 3.11.7 3.12.0
+      pyenv global 3.9.4 3.10.2 3.11.7 3.12.0
+      pyenv local 3.9.4 3.10.2 3.11.7 3.12.0
     fi
     # otherwise: just use the system python
     # avoid that git complains about dubious ownership if we use fakeroot: