ENH: Python 3.6 EOF

[tobiasraabe]

Is your feature request related to a problem?

Python 3.6's end of life will be 23th December 2021.

pytask will timely end support for it as well with the next minor version.

Todo

Here is a list of changes to phase out Python 3.6:

• Remove Python 3.6 from the pipeline.
• Set the --py37plus flag for pyupgrade.
• Remove _py36_windowsconsoleio_workaround.