Drop support for Python < 3.7

[prjemian]

Since Python support for version 2.7 was ended at the start of 2020, and support for Python 3 versions up to 3.6 ends at the end of this year, it is time for the NeXus definitions to drop its support for Python < 3.7. (See #946, for example.)

The processes that publish the documentation should be revised. The continuous integration workflow tests for these Python versions at this time:

definitions/.github/workflows/syntax-checks.yml

Line 11 in 493adbe

python-version: ['3.6', '3.7', '3.8', '3.9']

[PeterC-DLS]

six is still used in some of the code so perhaps we can remove the use in them

[prjemian]

At today's telco, we agreed that this issue should continue, to drop support for Python versions <3.7.

[prjemian]

See issue #953 (& PR #955) about the documentation publishing process.

[prjemian]

Much of this work was completed in PR #951. For this issue , examine the repo for any remaining references.