We need to manually specify these now so let's add a few that we'd
either previously documented or we've caught being used in the wild and
adjust the documentation accordingly.
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
```
This was accounted for in Version::NULL.<=>, but not in Version#<=>,
so these could only be compared if Version::NULL was the left hand
side.
The check had to go above the check that `other` is a version because
`Version::NULL`'s anonymous class does not inherit from `Version`.
(The type check feels like it's probably a smell, but out of scope).
