python3-testtools: upgrade 2.5.0 -> 2.6.0

python3-testtools: upgrade 2.5.0 -> 2.6.0

Improvements
------------
* Add support for Python 3.10 and 3.11.
* Drop support for Python 3.5 (EOL).
* Distutils integration is deprecated and will be removed in the next major
  version.
* Use "CompoundFixture" from "fixtures>=2.0" rather than rolling our
  own.
* Provide a "testtools[twisted]" extra documenting dependencies needed for
  "testtools.twistedsupport".
* Make "TestCase" hashable.
* Prevent "AttributeError" in "TestCase.__eq__" for objects lacking a
  "__dict__" attribute.
* Replace deprecated "cgi" module usage with "email".

Signed-off-by: Wang Mingyu <wangmy@fujitsu.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>