Update LibCST-based fixes for Python 3.15 (#28616)
Summary
--
This PR bumps our libcst version to 1.9 and incorporates the new support
for Python 3.15 into fixes
that involved imports and unpacking in comprehensions. The unpacking
fixes affect three C4 rules and are
in the first (non-empty) commit, while the lazy import fixes are a bit
more widespread and are in
the second commit.
I admittedly wasn't that familiar with our usage of libcst before this,
but it looks like we have
kind of a weird mix between using libcst and additional manual string
manipulation. My changes
preserve that with a couple of `format!("lazy {fix}")` calls. libcst
also itself makes this change a
bit awkward by using separate node types for lazy and non-lazy imports,
so I ended up returning the
parts of the imports instead of `Import` types directly in a couple of
cases.
Test Plan
--
Updated existing snapshots for the C4 rules and new mdtests for the
import helpers
