Fix Assimulo pip extra metadata

[parkyr]

Closes #109.

Summary

• Remove the unresolvable assimulo==3.4.3 PyPI requirement from the assimulo optional extra and requirements-assimulo.txt.
• Keep cython>=0.29,<3 as the pip-installable helper for source-build workflows.
• Document that Assimulo itself is supported through the conda-forge environment or a local source build, not through PyPI.

Related: #8 is packaging-modernization context only; this PR does not depend on it.

Validation

• uv pip install -e '.[assimulo]' --dry-run --python .venv/bin/python --cache-dir /tmp/pharmapy-uv-cache-issue109
• uv pip install -r requirements-assimulo.txt --dry-run --python .venv/bin/python --cache-dir /tmp/pharmapy-uv-cache-issue109
• .venv/bin/python -m pytest --collect-only
• .venv/bin/python -m pytest tests/ -m 'not assimulo'
• git diff --check

Branch Hygiene

• Base branch: master
• Branch point: origin/master@1a3ab3155216f07e9a18c60687a7b6b1c5959202
• Source branch: fix/issue-109-assimulo-extra
• Stacked status: not stacked; no prerequisite PRs
• Upstream status: upstream/master@400e1e13ad4ecd3ba202d5fae8c70ec73bdf9c3e is an ancestor of origin/master
• Open PR overlap: none found; open PRs Fix semibatch reactor energy flow term #102 and Fix crystallizer energy balance units and jacket paths #106-Fix drying gas mass-rate balance #108 touch source/test files unrelated to this packaging/docs change