ArgumentParser's add_argument returns the Action that was created.

[EvanKrall]

Fixes #1878

I added the member variables of Action to fix this error with mypy_selftest:

mypy/main.py:239: error: "Action" has no attribute "dest"

However, currently I'm getting:

mypy/main.py:240: note: unused 'type: ignore' comment

I suppose to fix this I would have to remove the type: ignore line in mypy (now that it's unnecessary), but it seems that would cause a chicken-and-egg problem: such a PR against mypy would fail tests until this PR is merged.

[JelleZijlstra]

Here's what I think we should do:

1. Merge this PR (after reviewing it, which I haven't done yet)
2. Make a PR to mypy that syncs typeshed and removes the now-unused type ignore
3. Merge it

[EvanKrall]

Running typeshed's mypy_selftest.py against the PR'd version of both mypy and typeshed results in:

mypy/main.py:235: error: Argument "dest" to "add_argument" of "ArgumentParser" has incompatible type "Optional[str]"; expected "str"

It appears that parser.add_argument(..., dest=None) works fine:

Python 2.7.10 (default, Feb  7 2017, 00:08:15)
[GCC 4.2.1 Compatible Apple LLVM 8.0.0 (clang-800.0.34)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import argparse
>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--blah', dest=None)
_StoreAction(option_strings=['--blah'], dest='blah', nargs=None, const=None, default=None, type=None, choices=None, help=None, metavar=None)

(same behavior in 3.6.1).

dest=None handled here:

• https://github.com/python/cpython/blob/2.7/Lib/argparse.py#L1421
• https://github.com/python/cpython/blob/3.6/Lib/argparse.py#L1462

I guess I'll also change dest to Optional[str].

[emmatyping]

Look forward to this and python/mypy#4578 :)