pip3 broken on emsdk python3 on macOS

Attempting to install `sphinx-build` via pip3 on macOS gives

```
MacBook-Pro:master clb$ pip3
-bash: /Users/clb/emsdk/python/3.7.4-2_64bit/bin/pip3: /usr/local/bin/python3.7: bad interpreter: No such file or directory
```
