vagrant: upgrade py 3.5.0 -> 3.5.3 due to openssl 1.1 compatibility

Vagrantfile

@@ -111,7 +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 3.5.3  # tests, 3.5.3 is first to support openssl 1.1<k
     pyenv install 3.6.8  # binary build, use latest 3.6.x release
     pyenv rehash
   EOF
@@ -191,8 +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 3.5.0 3.6.0 3.7.0
-      pyenv local 3.5.0 3.6.0 3.7.0
+      pyenv global 3.5.3 3.6.0 3.7.0
+      pyenv local 3.5.3 3.6.0 3.7.0
     fi
     # otherwise: just use the system python
     if which fakeroot 2> /dev/null; then