The `synstructure` docs say "This method is a no-op, underscore consts
are used by default now." The behaviour change occurred going from
`synstructure` version 0.13.0 to 0.13.1.
detects redundant imports that can be eliminated.
for #117772 :
In order to facilitate review and modification, split the checking code and
removing redundant imports code into two PR.
