flask web app部署到 heroku时，提示python 版本不兼容。heroku仅支持python3.6.3?

Question

我的flask web APP 虚拟环境是python 3.5.3;
runtime.txt:python3.5.3;requirements.txt和 Procfile文件都有；执行pip install -r requirements.txt没有报错。
然而当我使用命令：git push heroku master 部署时出现：
.....
--->Python app detected

The lastest version of python 3 is python-3.6.3(you are using Python-2.7.13,which is unsupported).
We recommend upgrading by specifying the latest version(python-3.6.3).
Learn more:https//devcenter.heroku.com/articles/python-runtimes

--->Installing Python-2.7.13

Requested runtime (Python-2.7.13) is not available for this stack(heroku-16).
Aborting. More info:https://devcenter.heroku.com/articles/python-support
Push failed
....

.....
请问这是怎么回事呢，我在这个虚拟环境中运行$:python -V, 确实显示的是python3.5.3,然后我改runtime.txt:python-3.6.3 还是报错，初次入坑 python flask web，请大家帮我看看，谢谢。