Vagrantfile: add python 3.5.0 for testing

Vagrantfile

@@ -111,6 +111,7 @@ def install_pythons(boxname)
     . ~/.bash_profile
     pyenv install 3.7.0  # tests
     pyenv install 3.6.0  # tests
+    pyenv install 3.5.0  # tests
     pyenv install 3.6.6  # binary build, use latest 3.6.x release
     pyenv rehash
   EOF
@@ -190,8 +191,8 @@ def run_tests(boxname)
     . ../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 3.6.0 3.7.0
-      pyenv local 3.6.0 3.7.0
+      pyenv global 3.5.0 3.6.0 3.7.0
+      pyenv local 3.5.0 3.6.0 3.7.0
     fi
     # otherwise: just use the system python
     if which fakeroot 2> /dev/null; then