Avoid calls to ast in plugins

[ericwb]

If one day we do wish to have the capability to swap the builtin Python ast for another parser, it's best to abstract any direct calls to the ast parser in plugins.

This change addresses calls from plugins and other functions to isinstance with ast node types as arguments. Further work is needed for other ast calls in subsequent patches.

Related to #894

Signed-off-by: Eric Brown [email protected]