no-op close method so that Scraper can be context manager

[fgregg]

right now

with scrapelib.Scraper(...) as scraper:
    ...

results in a NotImplemented error because when the context is exited, the session tries to close all the associated adapters and the FTPAdapter does not have the method.

[jamesturk]

thanks for this, if you have time to fix the small mypy issues that'd be great, otherwise glad to do it & merge this in soon 👍

[fgregg]

done!