Raise to debhelper version 12, use Python 3 and fix some bugs

* d/compat: Raise to 12.
* d/control (Build-Depends): Raise debhelper to version 12. Use python3-dev
  instead of python-dev (closes: #942912).
  (Standards-Version): Bump to 4.4.1.
  (Architecture): Limit to 64bit architectures (closes: #811449).