Fix flake8 warnings for sets and use PowerSet instead of None in .powerset

[sylee957]

References to other Issues or PRs

Brief description of what is fixed or changed

I've cleaned up some flake8 warnings for unused imports and variables.

Also, I've changed the behavior of .powerset to return PowerSet object instead of None for infinite sets like interval, which has better capability.

Other comments

Release Notes

• sets
  • Set.powerset will return PowerSet instead of None for infinite sets.

[sympy-bot]

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Here is what the release notes will look like:

• sets
  • Set.powerset will return PowerSet instead of None for infinite sets. (#17775 by @sylee957)

This will be added to https://github.com/sympy/sympy/wiki/Release-Notes-for-1.5.

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#### References to other Issues or PRs
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#### Brief description of what is fixed or changed

I've cleaned up some flake8 warnings for unused imports and variables.

Also, I've changed the behavior of `.powerset` to return `PowerSet` object instead of `None` for infinite sets like interval, which has better capability.

#### Other comments


#### Release Notes

<!-- Write the release notes for this release below. See
https://github.com/sympy/sympy/wiki/Writing-Release-Notes for more information
on how to write release notes. The bot will check your release notes
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<!-- BEGIN RELEASE NOTES -->
- sets
  - `Set.powerset` will return `PowerSet` instead of `None` for infinite sets.
<!-- END RELEASE NOTES -->

Update

The release notes on the wiki have been updated.

[oscarbenjamin]

Looks good