python/cpython

* Add new capability to the PEG parser to type variable assignments. For instance:
```
       | a[asdl_stmt_seq*]=';'.small_stmt+ [';'] NEWLINE { a }
```

* Add new sequence types from the asdl definition (automatically generated)
* Make `asdl_seq` type a generic aliasing pointer type.
* Create a new `asdl_generic_seq` for the generic case using `void*`.
* The old `asdl_seq_GET`/`ast_seq_SET` macros now are typed.
* New `asdl_seq_GET_UNTYPED`/`ast_seq_SET_UNTYPED` macros for dealing with generic sequences.
* Changes all possible `asdl_seq` types to use specific versions everywhere.

Partially revert commit ac46eb4:
"bpo-38113: Update the Python-ast.c generator to PEP384 (gh-15957)".

Using a module state per module instance is causing subtle practical
problems.

For example, the Mercurial project replaces the __import__() function
to implement lazy import, whereas Python expected that "import _ast"
always return a fully initialized _ast module.

Add _PyAST_Fini() to clear the state at exit.

The _ast module has no state (set _astmodule.m_size to 0). Remove
astmodule_traverse(), astmodule_clear() and astmodule_free()
functions.

…file.* functions (GH-21763)

`None` doesn't work:

```python
>>> import tempfile
>>> tempfile.TemporaryFile(buffering=None)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/sergey/tmp/cpython-dev/Lib/tempfile.py", line 607, in TemporaryFile
    return _io.open(fd, mode, buffering=buffering,
TypeError: 'NoneType' object cannot be interpreted as an integer
```

Automerge-Triggered-By: @vsajip