select_form: single return statement for clarity
select_form: single return statement for clarity

Slight modification of PR #192 to make the code logic a bit more
consistent between when the `selector` is a bs4 tag and when it
is a CSS selector.

No functionality changes.

hickford/MechanicalSoup,hemberger/MechanicalSoup,MechanicalSoup/MechanicalSoup