Remove support for Python 3.6

[davidhassell]

Python 3.6 is already on security patches only - no bug fixes - with an end-of-life in December 2021 (https://endoflife.date/python)

There are already two work-arounds to support Python 3.6 relating to:

• dict not being ordered at 3.6
• no re.Pattern at 3.6

Also:

• support for Python 3.6 has been dropped from numpy 1.20

I'm inclined to also drop support for 3.6 sooner than the end of the year.

This issue is inexorably linked to NCAS-CMS/cfdm#139, in which it has been decided to drop python 3.6 sooner than the end-of-life date, which means in practice that v3.9.0 will be the last cf release to support Python 3.6

[davidhassell]

It turns out we didn't drop Python 3.6 at v3.9.0, nor v3.10.0, but we will at v3.11.0!