TYP: Add typing.overload signatures to DataFrame.drop_duplicates.
This adds overloads so that a type checker can determine whether drop_duplicates returns DataFrame or None based on the value of the `inplace` argument.

The motivation for this is that we're using a type checker that looks at the pandas source code for type information. I realize that we should be using the stubs instead, but this will unblock us in the meantime and seems like an improvement to the type annotations anyway.
Thank you @rchen152! Thanks for the quick review and merge :)