travis: start testing 3.9 somewhat (#4071)

* travis: run stubtest on py39, allowing failures

* travis: run mypy with python version 3.9

* boto.compat, base64: fix version handling of (en|de)codebytes

* venv: fix #4010

Co-authored-by: hauntsaninja <>

.travis.yml

@@ -45,3 +45,8 @@ jobs:
       python: 3.5
       install: pip install -U git+git://github.com/python/mypy@0a05e61a
       script: ./tests/stubtest_test.py
+  allow_failures:
+    - name: "stubtest py39"
+      python: 3.9-dev
+      install: pip install -U git+git://github.com/python/mypy
+      script: ./tests/stubtest_test.py