build: remove unused docutils dependency

[nightcityblade]

Fixes #432

Removes docutils from the runtime dependency list in pyproject.toml. I checked the repository and it is not imported from package or test code, so keeping it as an install requirement appears unnecessary.

Validation:

• built sdist and wheel with python -m build
• installed the package into a fresh virtualenv with python -m pip install .
• verified installed metadata no longer lists docutils as a requirement
• verified import pdb2pqr succeeds after install