无法在Mac OS High Sierra版本10.13.5上安装virtualenv

Question

我正在运行Mac OS High Sierra版本10.13.5。我尝试安装virtualenv并收到消息说我需要将pip升级到更高版本

AKALBAG-M-82RZ:Python amkalbag$ pip install --user virtualenv
You are using pip version 6.1.1, however version 10.0.1 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
Collecting virtualenv
  Could not find a version that satisfies the requirement virtualenv (from versions: )
  No matching distribution found for virtualenv

所以我累了升级点数：

AKALBAG-M-82RZ:~ amkalbag$ pip install --upgrade pip
You are using pip version 6.1.1, however version 10.0.1 is available.
You should consider upgrading via the 'pip install --upgrade pip' command.
Requirement already up-to-date: pip in /Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages

我也尝试使用sudo，但这也无济于事。

AKALBAG-M-82RZ:~ amkalbag$ sudo -H pip install --upgrade pip
Password:
Requirement already up-to-date: pip in /Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/site-packages

我不断收到消息“需求已经是最新的”。

如何升级到pip版本10.0.1，以便随后安装virtualenv？