[iss623] Move pytest to development dependencies

[Dodothereal]

[iss623] Move pytest to development dependencies

Summary

pytest was declared as a runtime dependency in both package metadata and the requirements file, even though it is only used by the test suite. This removes it from normal installations and makes it available through the dev extra for contributors.

Fixes #623

Changes

• Remove pytest from install_requires and requirements.txt.
• Add pytest>=4.1.0 to the dev optional dependency extra.

Testing

Ran in the required sandbox:

python -m venv /tmp/brightwind-runtime && /tmp/brightwind-runtime/bin/python -m pip install --upgrade pip && /tmp/brightwind-runtime/bin/python -m pip install . && /tmp/brightwind-runtime/bin/python -c "import importlib.util; assert importlib.util.find_spec('pytest') is None" && /tmp/brightwind-runtime/bin/python -m pip install '.[dev]' && /tmp/brightwind-runtime/bin/python -c "import pytest; print(pytest.__version__)"

The normal package installation completed without pytest; installing .[dev] then installed and imported pytest successfully.

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