ImportError under Python 3.9

[Zac-HD]

Via Zac-HD/stdlib-property-tests#12, attempting to import libcst fails on Python 3.9 with

Traceback (most recent call last):
  File "t.py", line 3, in <module>
    import libcst
  File "libcst/__init__.py", line 187, in <module>
    from libcst._parser.entrypoints import parse_expression, parse_module, parse_statement
  File "libcst/_parser/entrypoints.py", line 25, in <module>
    _DEFAULT_PARTIAL_PARSER_CONFIG: PartialParserConfig = PartialParserConfig()
  File "<string>", line 8, in __init__
  File "libcst/_parser/types/config.py", line 132, in __post_init__
    raise ValueError(
ValueError: LibCST can only parse code using one of the following versions of
		    Python's grammar: 3.0, 3.1, 3.3, 3.5, 3.6, 3.7, 3.8. More versions may 
		    be supported by future releases.

Given that I'm using libcst in hypothesmith to test (among other things) pre-release Python versions, it would be lovely if libcst defaulted to the newest known grammar on newer-than-known Python versions rather than raising an error on import 😄

[edison12a]

I'm getting this error too when using MonkeyType
cc: @carljm

[edison12a]

What can we do to make the change happen?
Does it need testing? expansion?
@jimmylai

[Zac-HD]

Closed by #306.

(@thatch - the magic auto-close syntax only works in the PR description, not latter comments)