devel/py-intbitset: Enable support for Python 3.7 and 3.8

• Update to a more recent commit which contains a regenerated version of intbitset.c that was created with Cython 0.29.14 (not in Ports tree, yet).
  
  With the regenerated code the package can now also be built with Python 3.7 and 3.8.

PR: 233770 (related)