general: enable and fix flake8-isort issues

doc/conf.py

examples/from_firedrake.py

examples/mesh-to-tikz.py

examples/moving-geometry.py

examples/multiple-meshes.py

examples/parallel-vtkhdf.py

examples/plot-connectivity.py

examples/simple-dg.py

examples/to_firedrake.py

examples/tp-lagrange-stl.py

meshmode/__init__.py

meshmode/array_context.py

meshmode/discretization/__init__.py

meshmode/discretization/connection/__init__.py

meshmode/discretization/connection/chained.py

meshmode/discretization/connection/direct.py

meshmode/discretization/connection/face.py

meshmode/discretization/connection/modal.py

meshmode/discretization/connection/opposite_face.py

meshmode/discretization/connection/projection.py