Improve csv module and float() signature

[gvanrossum]

The most recent changes to the csv stubs (#1398) introduced more precise types but also some problems. I am fixing those problems here:

• DictReader accidentally became an abstract class. I made it concrete again by adding definitions for __iter__() and next().
• The restval argument to DictWriter.__init__() accidentally became mandatory. I made it Optional again.
• The first argument to reader() should be Iterable, not Iterator.

Running with these improvements against our internal codebase also revealed a problem when float() is called with an argument of type Union[str, int]. I fixed this by switching the signature of float.__init__() from using @overload to using Union.