Fix installer

[klntsky]

Changes in https://bootstrap.pypa.io/get-pip.py script made the installer unusable.

this_python = sys.version_info[:2]
min_version = (3, 6)
if this_python < min_version:
    message_parts = [
        "This script does not work on Python {}.{}".format(*this_python),
        "The minimum supported Python version is {}.{}.".format(*min_version),
        "Please use https://bootstrap.pypa.io/pip/{}.{}/get-pip.py instead.".format(*this_python),
    ]
    print("ERROR: " + " ".join(message_parts))
    sys.exit(1)

[klntsky]

This does not make the installer fully usable, though.

[kritiksoman]

Sorry we were not accepting PR as GIMP was transitioning to python3 and python2 was already deprecated. If you are interested, please contribute to this branch which is meant for python 3.