Remove dependency on improperly used and unmaintained contract module

- contract module has not see a release since 2010
- Corresponding PEP has been deferred
- python-contract module in Debian is orphaned
- We are only using contract module in one method of one module
- That can be replaced with one line of check instead of depending on an
  entire module
- The code using contract module does not work
- There is already replacement one line code that is actually working