Resolve mypy error in mcmc.py

[Ok3ks]

Description

Switching from list comprehension to np.fromiter converts the list to an NDArray which satisfies Mypy Type checks

Related Issue

• [ ] Closes #
• [ ] Related to #

Checklist

• [ ] Checked that the pre-commit linting/style checks pass
• [ ] Included tests that prove the fix is effective or that the new feature works
• [ ] Added necessary documentation (docstrings and/or example notebooks)
• [ ] If you are a pro: each commit corresponds to a relevant logical change

Type of change

• [ ] New feature / enhancement
• [ ] Bug fix
• [ ] Documentation
• [ ] Maintenance
• [x] Other (please specify):

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📚 Documentation preview 📚: https://pymc--7854.org.readthedocs.build/en/7854/