Disable forked and unpin pytest

[jaraco]

❓ What kind of change does this PR introduce?

• [ ] 🐞 bug fix
• [ ] 🐣 feature
• [ ] 📋 docs update
• [ ] 📋 tests/coverage improvement
• [ ] 📋 refactoring
• [ ] 💥 other

📋 What is the related issue number (starting with #)

Resolves #

❓ What is the current behavior? (You can also link to an open issue here)

❓ What is the new behavior (if this is a feature change)?

📋 Other information:

📋 Contribution checklist:

(If you're a first-timer, check out this guide on making great pull requests)

• [ ] I wrote descriptive pull request text above
• [ ] I think the code is well written
• [ ] I wrote good commit messages
• [ ] I have [squashed related commits together][related squash] after the changes have been approved
• [ ] Unit tests for the changes exist
• [ ] Integration tests for the changes exist (if applicable)
• [ ] I used the same coding conventions as the rest of the project
• [ ] The new code doesn't generate linter offenses
• [ ] Documentation reflects the changes
• [ ] The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences

[related squash]: https://github.com/todotxt/todo.txt-android/wiki/Squash-All-Commits-Related-to-a-Single-Issue-into-a-Single-Commit

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This change is 

[jaraco]

@webknjaz Per your request, I'll not merge this and I'll leave it to you to follow-up later.

[webknjaz]

@jaraco thank you!

[webknjaz]

FYI the tests fail because the pip wrapper in tox auto-injects the constraints into the pip install commands when the corresponding lock files exist. And since those pins aren't updated, this creates a conflict in dependency resolution.