Support PEP-614 - relaxed grammar restrictions on decorators

[Zac-HD]

Following PEP-614, the follow code is valid in Python 3.9 and later:

@(lambda f: f)  # or other expressions, e.g. `@buttons[0].clicked.connect`
def valid_on_py39():
    pass

but libcst.parse_module() raises

Traceback (most recent call last):
  ...
libcst._exceptions.ParserSyntaxError: Syntax Error @ 1:2.
Incomplete input. Encountered '(', but expected 'NAME'.

@(lambda f: f)
 ^

This syntax isn't in common use yet, but shed depends on LibCST for --refactor mode and I'd love to support it 😃

[zsol]

So this is already supported by the native parser (currently enabled by setting the LIBCST_PARSER_TYPE env var to native), but validation rules on the CST nodes disallow it. Needs a PR to relax the validation rules before this issue can be closed out

[Zac-HD]

Closed by #926 and fixed in version 0.4.10 🎉