Switch to Mercurial 5.9
Versions older than 5.9.2 cannot be built with pip with Rust extensions (see mercurial/mercurial-devel@58fe6d127a01).
Closes #541 (closed)
Versions older than 5.9.2 cannot be built with pip with Rust extensions (see mercurial/mercurial-devel@58fe6d127a01).
Closes #541 (closed)