当我使用pyenv virtualenv创建虚拟环境时，与之一起添加的pip的版本为19.0.3，但这已不是最新版本。我在源环境中安装了更新的pip，但这似乎并不影响在virtualenv中添加的版本。
$ pyenv virtualenv 3.7.3 test-env
Looking in links: /var/folders/hl/mm_f1p2j3pg211388dkh_wvm2fkj9r/T/tmpiasxckc0
Requirement already satisfied: setuptools in /Users/narunas/.pyenv/versions/3.7.3/envs/test-env/lib/python3.7/site-packages (40.8.0)
Requirement already satisfied: pip in /Users/narunas/.pyenv/versions/3.7.3/envs/test-env/lib/python3.7/site-packages (19.0.3)
$ pyenv shell test-env
(test-env) $ pip --version
pip 19.0.3 from /Users/narunas/.pyenv/versions/3.7.3/envs/test-env/lib/python3.7/site-packages/pip (python 3.7)
(test-env) $ pyenv shell 3.7.3
$ pip --version
pip 19.1.1 from /Users/narunas/.pyenv/versions/3.7.3/lib/python3.7/site-packages/pip (python 3.7)