[ospec] pinpoint the o.only() call site

[pygy]

Description

This adds a cleaned up stack trace after the o.only() warning.

I'll update the change log once #2147 is merged

Motivation and Context

Makes it easier to find the place you added that .only() call.

How Has This Been Tested?

Manually

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation change

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.
• I have updated docs/change-log.md

[barneycarroll]

Brilliant — that trick will do nicely for identifying the origin of async timeouts too.

[pygy]

Great minds think alike, I got the same thought when I saw #2159