Fixes#1850.
The fix was already applied to Python 2, but the typevar-based solution there
leads to "cannot infer value of type variable" in mypy. I used the following
script to check:
```python
from itertools import product
reveal_type(product([1]))
reveal_type(product([1], ['x'], [False], [3.0], [(1,)], [('x',)], [{1}], [{1: 2}], repeat=5))
```
Add an additional overload for the `repeat = n` case. Since we don't know
the numeric value of `repeat`, this just produces an iterator over an
arbitrary-length tuple.
From Samuel Freilich:
In Python 2, the predicate parameter in itertools.ifilter and
itertools.ifilterfalse can be None, indicating that true or false values
should be retained (functionally equivalent to passing "bool" as the
predicate). In Python 3, filter and itertools.filterfalse have
the same behavior.
Using izip with up to 6 arguments will retain the arguments' type information,
using izip with 7 and more arguments will discard type information about the
generated tuple items.
* Fix a few return types in stdlib/2/inspect.pyi.
* Rename _FrameRecord to _FrameInfo
* Correct some return types in itertools.pyi from Iterable to Iterator.
