vagrant: build 3.9.0rc2 via pyenv

3.9.0 should be available soon, then replace 3.9.0rc2 -> 3.9.0.

Vagrantfile

@@ -132,6 +132,7 @@ end
 def install_pythons(boxname)
   return <<-EOF
     . ~/.bash_profile
+    pyenv install 3.9.0rc2  # tests, version supporting openssl 1.1
     pyenv install 3.8.0  # tests, version supporting openssl 1.1
     pyenv install 3.7.0  # tests, version supporting openssl 1.1
     pyenv install 3.6.10  # binary build, tests, version supporting openssl 1.1
@@ -214,8 +215,8 @@ 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 3.6.10 3.7.0 3.8.0
-      pyenv local 3.6.10 3.7.0 3.8.0
+      pyenv global 3.6.10 3.7.0 3.8.0 3.9.0rc2
+      pyenv local 3.6.10 3.7.0 3.8.0 3.9.0rc2
     fi
     # otherwise: just use the system python
     if which fakeroot 2> /dev/null; then