virtualenv_create(libexec, "python") results in error upon install

When the line was switched to the more explicit

`virtualenv_create(libexec, "python3")` the package installed with any issues. 

Arguably, I am not as comfortable as I would like with Homebrew's relationship with Python, so maybe this reveals a larger underlying issue or there was a mistake within the original Formula.

The formula is hosted [here](https://github.com/saulpw/homebrew-vd). 

```
Last 15 lines from /Users/anja/Library/Logs/Homebrew/visidata/14.pip:
  Removing source in /private/tmp/pip-req-build-CH6VRn
visidata requires Python '>=3.4' but the running Python is 2.7.10
Exception information:
Traceback (most recent call last):
  File "/usr/local/Cellar/visidata/1.2/libexec/lib/python2.7/site-packages/pip/_internal/basecommand.py", line 228, in main
    status = self.run(options, args)
  File "/usr/local/Cellar/visidata/1.2/libexec/lib/python2.7/site-packages/pip/_internal/commands/install.py", line 291, in run
    resolver.resolve(requirement_set)
  File "/usr/local/Cellar/visidata/1.2/libexec/lib/python2.7/site-packages/pip/_internal/resolve.py", line 103, in resolve
    self._resolve_one(requirement_set, req)
  File "/usr/local/Cellar/visidata/1.2/libexec/lib/python2.7/site-packages/pip/_internal/resolve.py", line 262, in _resolve_one
    check_dist_requires_python(dist)
  File "/usr/local/Cellar/visidata/1.2/libexec/lib/python2.7/site-packages/pip/_internal/utils/packaging.py", line 55, in check_dist_requires_python
    '.'.join(map(str, sys.version_info[:3])),)
UnsupportedPythonVersion: visidata requires Python '>=3.4' but the running Python is 2.7.10
```

docs/Python-for-Formula-Authors.md

@@ -66,7 +66,7 @@ This is exactly the same as writing:
 ```ruby
 def install
   # Create a virtualenv in `libexec`. If your app needs Python 3, make sure that
-  # `depends_on "python"` is declared, and use `virtualenv_create(libexec, "python")`.
+  # `depends_on "python"` is declared, and use `virtualenv_create(libexec, "python3")`.
   venv = virtualenv_create(libexec)
   # Install all of the resources declared on the formula into the virtualenv.
   venv.pip_install resources