fix DictReader definition (#1475)

* constructable with Iterable
* __iter__ returns another DictReader
* supports `__next__` on python 3
* supports `next` on python 2

Author: graingert

stdlib/2and3/csv.pyi

@@ -59,11 +59,11 @@ if sys.version_info >= (3, 6):
         dialect = ...  # type: _Dialect
         line_num = ...  # type: int
         fieldnames = ...  # type: Sequence[str]
-        def __init__(self, f: Iterator[str], fieldnames: Sequence[str] = ...,
+        def __init__(self, f: Iterable[str], fieldnames: Sequence[str] = ...,
                      restkey: Optional[str] = ..., restval: Optional[str] = ..., dialect: _Dialect = ...,
                      *args: Any, **kwds: Any) -> None: ...
-        def __iter__(self) -> Iterator[OrderedDict[str, str]]: ...
-        def next(self) -> OrderedDict[str, str]: ...
+        def __iter__(self) -> 'DictReader': ...
+        def __next__(self) -> OrderedDict[str, str]: ...
 else:
     class DictReader(Iterator[Dict[Any, str]]):
         restkey = ...  # type: Optional[str]
@@ -72,11 +72,14 @@ else:
         dialect = ...  # type: _Dialect
         line_num = ...  # type: int
         fieldnames = ...  # type: Sequence[str]
-        def __init__(self, f: Iterator[str], fieldnames: Sequence[str] = ...,
+        def __init__(self, f: Iterable[str], fieldnames: Sequence[str] = ...,
                      restkey: Optional[str] = ..., restval: Optional[str] = ..., dialect: _Dialect = ...,
                      *args: Any, **kwds: Any) -> None: ...
-        def __iter__(self) -> Iterator[OrderedDict[Any, str]]: ...
-        def next(self) -> OrderedDict[Any, str]: ...
+        def __iter__(self) -> 'DictReader': ...
+        if sys.version_info >= (3,):
+            def __next__(self) -> OrderedDict[Any, str]: ...
+        else:
+            def next(self) -> OrderedDict[Any, str]: ...
 
 class DictWriter(object):
     fieldnames = ...  # type: Sequence[str]