Log Message:
py-testrepository: updated to 0.0.21

0.0.21

CHANGES

* 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. (Robert Collins)
* pbr is now used to manage build and release tooling.
* 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.