Ensure blib2to3.pygram is initialized before use

[Zac-HD]

For the last week or so, I've had a frustratingly irreproducible problem in Hypothesis CI, which gives me

INTERNALERROR>     pretty = black.format_str(unformatted, mode=mode)
INTERNALERROR>   File "src\black\__init__.py", line 1224, in format_str
INTERNALERROR>   File "src\black\__init__.py", line 1264, in _format_str_once
INTERNALERROR>   File "src\black\nodes.py", line 159, in visit
INTERNALERROR>   File "src\blib2to3\pytree.py", line 43, in type_repr
INTERNALERROR> ImportError: cannot import name 'python_symbols' from 'blib2to3.pygram' 

I'm now pretty sure that this is because blib2to3.pytree.type_repr() tries to import pygram.python_symbols, but this name is created by the initialize() function and that apparently hasn't been called yet. Which implies that black.nodes hasn't been imported, which is pretty surprising to me, but 🤷. Anyway, I'm working around this downstream regardless, but figured that I'd open a PR rather than a plain issue in case this was a helpful fix.

Thanks again for maintaining Black!

[github-actions]

diff-shades reports zero changes comparing this PR (8ab7a22) to main (a201003).

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[JelleZijlstra]

I wonder if this is a mypyc issue, though then I'm not sure how it would be inconsistent.

This PR seems simple enough so I'm inclined to accept it.

[Zac-HD]

Thanks @JelleZijlstra - I think if you apply the 'skip news' label this will be mergeable?