Fix CI due to subtests -- pytest incompatibility

[b-butler]

Description

Fixes CI tests from pytest >= 6.0 and pytest-subtest incompatibility see
pytest-dev/pytest-subtests#31.

Motivation and Context

It is nice not to have broken CI

Types of Changes

• Documentation update
• Bug fix
• New feature
• Breaking change¹

¹The change breaks (or has the potential to break) existing functionality.

Checklist:

• I am familiar with the Contributing Guidelines.
• I agree with the terms of the Contributor Agreement.
• My name is on the list of contributors.
• My code follows the code style guideline of this project.
• The changes introduced by this pull request are covered by existing or newly introduced tests.

If necessary:

• I have updated the API documentation as part of the package doc-strings.
• I have created a separate pull request to update the framework documentation on signac-docs and linked it here.
• I have updated the changelog.

[bdice]

lgtm. Thanks for the quick fix. We'll want to think about removing pytest-subtests in the future since its development tends to lag pytest and seems to break often.

[mikemhenry]

LGTM

[csadorf]

All of our test requirements should be fully pinned.

[bdice]

@csadorf I agree, we should have a specific version pinning. I have raised issue #332 to address that, and document our plan to remove pytest-subtests.