da/api.py: Fix file access mode.
Replace 'codegen' with 'unparse'; fix missing 'Expr' nodes in generated Python.

'unparse.py' is distributed with the Python source code and supports the
latest Python versions.

* da/compiler/pypr.py: removed.

* da/compiler/ui.py: use 'unparse'.

* da/compiler/incgen.py: use 'unparse'; fix missing 'Expr' nodes.

* da/compiler/pygen.py:  use 'unparse'; fix missing 'Expr' nodes.
incgen: fix 'delete' statements.

* da/compiler/pygen.py(PythonGenerator.visit_DeleteStmt): fix typo 'tgt'
  -> 'targets'.