Removing compatibility for Python 2

[jaraco]

In #1458 and the Setuptools 45 release, this project dropped declared support for Python 2, adding a warning when a late version was invoked on Python 2. This warning helped address many of the systemic uses of Setuptools 45+ on Python 2, but there continue to be users (presumably) reporting that they've encountered the warning.

I say presumably because most of them have submitted a blank template without providing any information.

Since March, these users have been directed to the template via bit.ly, so I have metrics on the number of users encountering and following the link.

It seems there have been 50-100 clicks per day since Apr 11. I'm guessing bit.ly doesn't give me data older than 30 days.

To put that in perspective, Setuptools received over 45M downloads in the last month, so the number of people that followed that link (3.3k) is 0.007% of the downloads.

Still, that's upwards of 100 people per day whose workflow would be broken until they could fix their environment.

Let's also consider that each of these users encountering this issue are following discouraged if not deprecated workflows and are creating new or updated environments (new since setuptools 45 was released in January).

It seems to me we have two options - support Python 2 until the incidents of users encountering this error message reduces to a trickle (what is that threshold) or bite the bullet and drop support for Python 2.

I'd like to review the outstanding issues relating to this issue, but my inclination is to move forward with dropping support.