创建虚拟环境没有成功：
python3 -m venv venv
错误提示：
The virtual environment was not created successfully because ensurepip is not
available. On Debian/Ubuntu systems, you need to install the python3-venv
package using the following command.
apt-get install python3-venv
You may need to use sudo with that command. After installing the python3-venv
package, recreate your virtual environment.
查看本地python版本为3.4
解决办法：
sudo apt-get install python3.4-venv
Then use this:
python3 -m venv venv
