Change Details

Reasons: * Currently `Site` does not appear in the generated documentation when we use `help(pywikibot)`. * Some IDEs that have intellisense feature, including IDLE, depend on the function to be available in `__all__` variable, otherwise won't suggest the related code completion. * Using `from pywikibot import *`does not import `Site` which seems to be an essential part of the package and should be imported as well. Although this kind of import should usually be avoided, it is okay to use it to save typing in interactive sessions.