rust: Make the hg-cpython crate default to Python 3
This default is used when running `cargo` manually such as for `cargo test`.
`setup.py` and `Makefile` both configure the Python major version explicitly.
Differential Revision: https://phab.mercurial-scm.org/D11618
edition = "2018"
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wrap_comments = true
error_on_line_overflow = true