- update to 0.0.21:
* testrepository now uses ``hatch`` rather than ``pbr``.
* setup.py testr now supports different coverage names.
* Isolate the testrepository test suite from the chunking (or
otherwise) behaviour of testtools' exception handlers.
* Unexpected errors reading streams from a file repository now
propogate the correct exception.
* testrepository now requires at least version 0.0.11 of the
subunit Python module.

- fix requires
- tighten runtime requires
- add buildrequires to satisfy test suite run
- Add BuildRequires: python-extras