Get mypy-clean, enable mypy in CI

.github/workflows/ci.yml

.gitlab-ci.yml

benchmarks/bench_translations.py

pyproject.toml

sumpy/__init__.py

sumpy/array_context.py

sumpy/assignment_collection.py

sumpy/codegen.py

sumpy/cse.py

sumpy/derivative_taker.py

sumpy/distributed.py

sumpy/e2e.py

sumpy/e2p.py

sumpy/expansion/__init__.py

sumpy/expansion/diff_op.py

sumpy/expansion/level_to_order.py

sumpy/expansion/local.py

sumpy/expansion/loopy.py

sumpy/expansion/m2l.py

sumpy/expansion/multipole.py