Kind inference: do not treat sums specially

Fortran self dep in loops

- do funcall isolation on AST
- do self dep isolation on AST
- do if isolation on AST
- implement ForLoop node codegen in Fortran
- add selfdep-in-loop test

https://github.com/inducer/modepy/pull/38#issuecomment-846472970

Doc improvements

Run flake8 in Github CI on target Py version