Adds warning about ongoing problems wit PIP 20.3 to bug template

[potiuk]

Part of #12838

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[kaxil]

I don't think we need this in the template.

The one pinned is explicit and grabs attention -- good work on that

[potiuk]

Let's see how the story evolves. I am afraid, that when we start announcing 2.0 and 1.10.14, we will be flooded with "Airflow does not install" issues.

And the more we protect against it, the better. In several places (unless of course pypa/pip#9231 or similar workaround gets implemented).

I also would like all of us, committers to be aware that we have those problems with PyPI, so I do not end up as a person handling all such PyPI -related issues like #12824. I got redirected to as 'local PIP expert there", but honestly, I do not feel like a person that all the "PIP 20.3 installation of Airflow is broken" should be directed to (or to any of the committers with that respect).

IMHO, the more vocal we are about it and proactively we tell about the problem the more likely our users will notice and downgrade PIP without involving slack conversations or opening issues.

I just do not want to say "we've done enough to warn our users" situation like PIP did. Surely, we can blame our users for not reading the docs, but this is the last thing I would do.