Improve Python 2 deprecation message

[pradyunsg]

The error message that pip prints when run on Python 2.7 is not very informative about what timeline pip would use before dropping support or how it might approach it.

Context: https://discuss.python.org/t/packaging-and-python-2/662/5?u=pradyunsg and #6231

[pradyunsg]

Adding to 19.2 since it is probably the best timed release for a better deprecation message, we've decided how we're going to handle post-core-developer-support and well, because I really want us to have clear messaging for deprecation as early as possible.