MAINT: flake8 updates

[jklenzing]

Description

There's a new version of flake8! Two major changes that I can tell:

• Specifying specific errors to ignore for files in setup.cfg no longer works (see error here)
• A number of additional flake8 errors are popping up (mostly unused imports, this may have slipped under the radar due to changes in how setup.cfg was processed with flake8 6.0)

This updates the issues throughout the code. Major changes include

• Adding comments for pysat/__init__ to explain why imports and code are mixed, including updating the logical flow a bit
• The file-level ignore statements have been moved to the files as line-level noqa statements (as is the standard elsewhere in the code)
• Most of the "unused imports" that are removed are related to tests on deprecations (removed in this version). Not really sure why flake8 didn't catch this with 6.0.
• Cleaned up some code duplication where proper style exceeded 80 chars (need space after ',')

Note that flake8 is expected to fail in all other branches.

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

running flake8 and pytest locally.

Test Configuration:

• Operating system: Ventura 13.6.1
• Version number: Python 3.10.9

Checklist:

• [rc] Make sure you are merging into the develop (not main) branch
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• Add a note to CHANGELOG.md, summarizing the changes

If this is a release PR, replace the first item of the above checklist with the release
checklist on the wiki: https://github.com/pysat/pysat/wiki/Checklist-for-Release