ValueError: Invalid Version range

I have 2 packages, A and B. Package A contains:
```python
depends_on('python@2.3:')
depends_on('B')
```
Package B only works with Python 2, and contains:
```python
depends_on('python@:2')
```
As far as I know, this should be valid, and Spack should concretize to the latest version of Python 2. Instead, I get the following error:
```
==> Error: Invalid Version range: 2.3:2
```