Issue 29622: ast module doesn't support optional fields of Parser/Python.asdl

The _ast.AST constructor requires a tuple with the same number of items than the fields list.

Example with Raise:

ASDL: Raise(expr? exc, expr? cause)

>>> import _ast
>>> _ast.Raise()
<_ast.Raise object at 0x7fc2ca7dee90>
>>> _ast.Raise(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: Raise constructor takes either 0 or 2 positional arguments
>>> _ast.Raise(1, 2)
<_ast.Raise object at 0x7fc2ca7def60>

The cause field is optional in the ASDL, but required in the _ast module.

A tradeoff would be to add the minimum and maximum number of fields to _ast.AST.

This issue should prevent API breakage caused by the new docstring attribute added by issue #29463.

Duplicating my comment of 29463 who was in a race condition with this issue:

----
thank you for your work on the AST, I know many developers are looking forward to improvement and stabilisation with the hope of having it stable (and documented) in the stdlib at some point. 

The recent change in PR 46 now change (at least) the constructor of `ast.Module` to take a second mandatory parameter (the docstring). 

I know the ast is autogenerated and "use at your own risk". But IPython for example, use `mod = ast.Module([nodes])`, with the second mandatory parameter added to Module that make it incompatible with current Python 3.7. 
Well it's long until it's released, and we can patch things, but I'm sure we are not the only one in this case, and we'd like older version of IPython to still be compatible with Python 3.7, so if  `Module()`'s second parameter (the docstring) could be optional, that would be great. 

I would be happy if it raise a deprecation warning that it will be required in the future.

I'm of course speaking about `Module` because that's the first error I encountered, but I'm guessing it applies to other changed AST nodes. 

Thanks.
---

AST type doesn't have any information about optional fields.  And I don't know it's worth enough to add it.
When AST is instantiated by keyword arguments, there are no check about absence of some fields.
I suppose we can just loosen positional arguments too.

current:

    if (PyTuple_GET_SIZE(args) > 0) {
        if (numfields != PyTuple_GET_SIZE(args)) {
            PyErr_Format(PyExc_TypeError, "%.400s constructor takes %s"
                         "%zd positional argument%s",
                         Py_TYPE(self)->tp_name,
                         numfields == 0 ? "" : "either 0 or ",
                         numfields, numfields == 1 ? "" : "s");

will be:

    if (PyTuple_GET_SIZE(args) > 0) {
        if (numfields > PyTuple_GET_SIZE(args)) {
            PyErr_Format(PyExc_TypeError, "%.400s constructor takes at most "
                         "%zd positional argument%s",
                         Py_TYPE(self)->tp_name,
                         numfields, numfields == 1 ? "" : "s");

Now trailing optional fields are optional arguments of AST type.

@mbussonn With PR 249, "import os" and "%timeit" works fine.

New changeset 4c78c527d215c37472145152cb0e95f196cdddc9 by INADA Naoki in branch 'master':
bpo-29622: Make AST constructor to accept less than enough number of positional arguments (GH-249)
https://github.com/python/cpython/commit/4c78c527d215c37472145152cb0e95f196cdddc9