MAINT: pytest syntax

[jklenzing]

Description

Addresses pysat/pysat#1053

updates pytest syntax (removes nose backwards-compatibility)

Type of change

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

How Has This Been Tested?

github actions, checking coverage.

Checklist:

• 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

[rstoneback]

Looks good. Thanks @jklenzing.