Temporary workaround for invalid install layout

* Patch by Nicolas Boulenguez, thanks!
* Temporary workaround to avoid shipping headers in /usr/include/python3.7
  (instead of /usr/include/python3.7m). This is likely to be a
  python3-stdlib-extensions bug so we should consider removing this as soon
  as the bug as been addressed (Closes: #920337).
* Migrate to compat = 12:
* debian/control:
  - update debhelper dependency and migrate to debhelper-compat.
  - Rules-Requires-Root: no.
* Remove now useless debian/compat file.
* Bump debian/watch version to 4.