Drop support for soon-EOL Python 3.6

[hugovk]

Changes proposed in this pull request:

• The Python 3.6 EOL is scheduled for 2021-12-23, before Pillow 9.0.0's 2022-01-02 release

  • https://devguide.python.org/#status-of-python-branches
  • https://endoflife.date/python

• Ran pyupgrade **/*.py --py36-plus to upgrade Python syntax

• No changes running pyupgrade **/*.py --py37-plus (other than removing stuff that will be removed for PILLOW_VERSION in another PR)

• Remove Docker jobs which test on Python 3.6:

  • centos-7-amd64
  • centos-8-amd64
  • centos-stream-8-amd64
    • Perhaps we could upgrade to 3.7 in a CentOS
    • Or check when CentOS Stream 9 is out and what Python it'll have
  • ubuntu-18.04-bionic-amd64
    • If we want to test 18.04, we can add it add ubuntu-18.04 as an include: to the regular test.yml
  • (Updated to use 3.9: Compile Python 3.9 instead of using package Python 3.6 docker-images#123 / Restored Docker jobs hugovk/Pillow#76)

Not removed the if sys.version_info >= (3, 7): stuff around the PILLOW_VERSION warnings, will remove that in the next PR. (Now removed, PILLOW_VERSION since removed in #5776.)

[cclauss]

Why pyupgrade --py36-plus and not pyupgrade --py37-plus?

[hugovk]

Why pyupgrade --py36-plus and not pyupgrade --py37-plus?

I ran pyupgrade --py36-plus first, then pyupgrade --py37-plus second, and pyupgrade --py37-plus made no other changes, other than removing some if sys.version_info >= (3, 7): guard that I'll be removing more fully in another PR to remove that whole bit.