If I know all versions of Python, the easiest way for me to look at all of the Python questions isn't for me to come up with some complicated search that makes sure I'm including all the possible Python tags. The easiest way is for all of the Python questions to have the python tag. The version specific tag (python-2.7) just provides the additional detail that the question (and answers) should be compatible with the 2.7 version of Python.
A question would be better off tagging python, omitting python-2.7, and then specifying that requirement in the body of the question than it would to have the python-2.7 tag and omit the python tag.