contrib: allow trailing commas in lists
black manages this for us, so we don't need to worry about this anymore.
Differential Revision: https://phab.mercurial-scm.org/D6977
contrib: remove check-code line length limit
Both Python and C code are managed by formatters now, so relax this.
Differential Revision: https://phab.mercurial-scm.org/D6976