Log message:
py-libcst: download peg-git during fetch stage, and provide it during build

avoids network access during build

Log message:
lang/py-libcst: import py-libcst-0.4.1

Packaged by adam and myself in wip.

LibCST parses Python 3.0 -> 3.11 source code as a CST tree that
keeps all formatting details (comments, whitespaces, parentheses,
etc). It's useful for building automated refactoring (codemod)
applications and linters.

LibCST creates a compromise between an Abstract Syntax Tree (AST)
and a traditional Concrete Syntax Tree (CST). By carefully reorganizing
and naming node types and fields, we've created a lossless CST that
looks and feels like an AST.