How to write Python code that is able to properly require a minimal python version?

You can take advantage of the fact that Python will do the right thing when comparing tuples:

#!/usr/bin/python
import sys
MIN_PYTHON = (2, 6)
if sys.version_info < MIN_PYTHON:
    sys.exit("Python %s.%s or later is required.\n" % MIN_PYTHON)

You should not use any Python 2.6 features inside the script itself. Also, you must do your version check before importing any of the modules requiring a new Python version.

E.g. start your script like so:

#!/usr/bin/env python
import sys

if sys.version_info[0] != 2 or sys.version_info[1] < 6:
    print("This script requires Python version 2.6")
    sys.exit(1)

# rest of script, including real initial imports, here

Starting with version 9.0.0 pip supports Requires-Python field in distribution's metadata which can be written by setuptools starting with version 24-2-0. This feature is available through python_requires keyword argument to setup function.

Example (in setup.py):

setup(
...
   python_requires='>=2.5,<2.7',
...

)

To take advantage of this feature one has to package the project/script first if not already done. This is very easy in typical case and should be done nonetheless as it allows users to easily install, use and uninstall given project/script. Please see Python Packaging User Guide for details.