CI failures: click has dropped support for Python 2.7

[licquia]

From the release notes for click (https://click.palletsprojects.com/en/8.0.x/changes/#version-8-0-0):

• Drop support for Python 2 and 3.5.

This means that all Python 2.7 tests fail in CI.

We should tie the click version to the Python version, so click 7.x is forced for Python 2.7.

[licquia]

Strictly speaking, we should probably do the same for Python 3.5 as well.

[pombredanne]

Click-wise 6.7 and up should be supported. 7.0 is known to be buggy. As a library it is best to keep ranges open enough.

[licquia]

The PR ties click only for 2.7; I verified that click 8.0.1 is still getting installed for Python 3.5 and newer.