Upgrade testtools on Python 3

testtools 2.5.0 dropped its dependency on unittest2, which is useful
since that module no longer works as of Python 3.10.

(It would be simpler to drop support for Python < 3.6 and upgrade
fixtures to at least 4.0.0, which would allow removing the testtools and
unittest2 requirements entirely; but I didn't want to do that in a
bug-fix branch.)

test-requirements.txt

@@ -11,6 +11,7 @@ mock==1.0.1
 nose2==0.13.0
 pbr==3.1.1
 python-mimeparse==1.6.0
-testtools==2.3.0
+testtools==2.3.0; python_version<"3"
+testtools==2.5.0; python_version>="3"
 traceback2==1.4.0
-unittest2==1.1.0
+unittest2==1.1.0; python_version<"3"