pip - ImportError: cannot import name main

I used `asdf global python system`; I have pip installed in `~/.local`:

```
$ pip -V
pip 19.0.1 from /home/roger/.local/lib/python2.7/site-packages/pip (python 2.7)
```

But when I change to a directory which uses a `local` python version (2.7.15), `pip` no longer works:

```
$ cd src/foo
$ pip -V
Traceback (most recent call last):
File "/home/roger/.asdf/installs/python/2.7.15/bin/pip", line 7, in
from pip import main
ImportError: cannot import name main
```